I almost always buy kits for subject matter, but kit price also tends to be a factor. Sometimes I don't have the cash to play with sometimes so I'll try to get something cheap. I tend to stick with cars mostly, but I've got a few starships and some anime girls. Box art doesn't matter as much as kit accuracy and quality. I'm perfectly happy buying a kit without a box. So far the only thing I dont care for on model building is painting the body. I dont have a set place to paint the body or set up an airbrush and I suck at spray painting, so I end up having to strip them pretty often.
As far as real vehicles I would like to have. I'd want two of my old cars back: my 1972 Buick Skylark hardtop and my 1978 Monte Carlo. I'd also like to have a couple of my brother's cars, an '81 Firebird and a 1971-1973 Dodge Charger. Beyond that, maybe a "land yacht" Cadillac, like a 1973 Brougham.