Hi Matt, not sure if by "looking for" you mean available in plastic / resin, or you mean ideas for 3D printing. If the second...
A Cummins L10 would probably be well received. It was a popular medium duty diesel truck engine in the 80s and 90s and often used in a fire engines. The Navistar DT466 and 530 series is another I know many have mentioned wanting. I know both have been on the to do list by some resin casters, but it has been years and I don't believe either have actually made it past the "to do" point.
Detroit diesels are always a possibility. Someone did a Detroit 6-71 but I don't think it was available long, it has been years since I saw it offered. The 6-71 (and related 2-71, 3-71, 4-71) was in use for 50+ years, extremely popular in medium and heavy duty trucks as well as marine and industrial use. The Detroit 6v71 / 6v92 can be made from an 8v71 / 8v92 but I would guess it would still do well as many would prefer to buy a ready to go engine kit vs converting one. I don't believe anyone has ever done a Detroit 6v53. It is another medium duty truck engine, that also saw a lot of use by the US military in light armored vehicles. There are some people out there now putting these engines into pickup trucks as it is about the biggest diesel that you can squeeze under a 1 ton hood. The Detroit 8.2 liter "fuel pincher" was by most accounts a terrible, terrible engine, but it was widely used in medium duty trucks, the Ford C and Louisville in particular.
A Caterpillar 3208 would be another medium duty to consider. Either a full engine or parts to do a conversion of the Caterpillar 1160 in the Ford Louisville kit (minor differences, mostly fuel rail and valve covers).
Some more obscure possibilities are some of the English diesels that found use in US trucks in the 1960s. The Perkins 6.354 which was used in several Dodge medium duty trucks including the L700. Ford of England had the Dagenham and Dorsett diesels which Ford used in the Ford C and F series trucks prior to 1970.
If you go to the Shapeways site they have links to a bunch of 3d programs that are compatible with Shapeways, free, cheap and pay programs. http://www.shapeways.com/creator/tools
I've been fooling around with Google Sketchup for a couple of years. There are a lot of good videos on youtube and I found it is fairly user friendly. Unfortunately I'm also finding it has limitations when it comes to 3d printing. It was originally designed for 3d modeling online, not really intended for 3d printing. They also keep some of the most useful features out of the free part. They do offer an inexpensive student version if you are able to get your teenager to remember to bring home his student ID (3 years running mine has been unable to manage this feat). I am in the process of trying some of the other programs, but none are what I'd consider intuitive.
There are actually a lot of good performance four cylinder engines, it's just that many fixate on the large displacement V8s. Turbo 4s were giving the gutted, smog / fuel consumption compliant V-8s of the 1980s a good run for performance. Of course there are a lot of great 4 cylinders doing what they do best, be lightweight, reliable and sip gas, but I don't think most are looking for those here.
Don't forget the Ford 2.3 liter used in the Pinto was later equipped with a turbo for use in the Merkur XR4Ti, Thunderbird Turbo Coupe and Mustang SVO where the final versions reached 200 hp and provided more power than the factory stock 5.0 V-8s. The 4 cyl SVO was the hottest Mustang available in the mid to late 80s. Volvo's 2.1 to 2.3 liter "red block" is legendary for its durability and ease of repair. It is not at all uncommon to find original engines still running well with 500,000+ miles on them. The non-turbos peaked at around 114hp, but the turbo versions in the late 80s were reaching 160-190 hp in stock form which was pretty respectable power for the time. 1983-85 Volvo offered a limited production (500 / year) evolution version of the 242 to allow them to compete in the European Touring car Championship. These cars made 300hp from a worked over redblock. Today enthusiasts are tweaking these engines to get 200-250hp, and heavily worked over examples nudging 400hp are out there. Not bad for a car that takes styling cues from Frigidaire.
My wife and I are getting a Silhouette Cameo for Christmas, so I haven't played with it yet. We both have hobbies that it seemed useful for, and they had a Black Friday deal on these a few weeks back. In addition to cutting thin plastic I know of aircraft modelers using these to create canopy masks and painting stencils to use in place of decals. With ALPS printers being increasingly difficult to get, a stencil could be a useful option.
Just pointing out that Silhouette and Cricut are brand names, they each offer multiple machines. The Portrait is the lower end machine offered by Silhouette (pricewise lines up with the mid range Cricut Explore Air machine at $179). Silhouette also has the Cameo 3 which compares to the top Cricut Explore Air 2 machine (both $249 with similar standard features). Cricut 's entry level Explore One machine is $139 (no comparable machine from Silhouette). The Portrait has an 8x12" mat size, the Cameo 3 and all of the Cricut machines have a 12x12" mat. The Silhouette Cameo and Cricut offer an optional 12x24" mat. The Silhouette Cameo 3 does offer an "auto blade" so you don't have to set the blade depth manually, but the manual blades remain an option. The Silhouette Cameo 3 and Cricut Explore Air 2 both offer a dual blade option. Both brands offer standard and "deep cut" blades. The Silhouette machines offer a pen conversion so the machine can draw instead of cut. Potentially useful for marking thicker materials, or maybe even for use with decal film. Silhouette offers a combination cutter / embossing machine (the Curio) which appears to be based on the Portrait (uses the same 8x12" mat).
I only point this out because your comparison may not be apples to apples, your friend may have the top end Cricut blinged out with all the options.
So many possibilities, SWAT or surveillance van, rescue or hazmat truck, roach coach, utility company truck...
Early in my career we had a broken down old 3/4 ton Chevy step van as a hazmat truck. It had been well used as a US Navy flight line repair truck before being passed on to us. If you were able to get the poor thing up to 45 mph it would start to wallow badly. That truck is probably being used to sell tacos in Salinas now if it hasn't rusted away completely. Mmmmmm tacos...
It would be nice to have all the options in one box. Most of my project ideas are dual drive day cabs, buying one of each kit to do that leaves me with parts for a single drive sleeper cab, kind of an unusual beast.
Yeah, a lot of these have been posted on the site. It is a fairly simple kit but a nice one with potential. If one was motivated it could be detailed by cutting out the engine bay and adding a full detail engine, it is also a good source for 4x4 parts.
I found it a fun kit, a great break from the overly ambitious projects many of us take on.