Dont know about Plaza Japan.com but they seem pretty reliable...As for me,I bought kits on ebay from either rc.sausage.dog or 1sthobbymodel (dont remember wich one it was),but these came packed in boxes with Plaza japan tags on it. The weird thing is that my items were shipped from hong kong and NOT from japan.
So, I guess Plaza Japan is like a big retailer for the asian market,can't be sure about it though...Anyways,I suggest you give it a try by buying only one kit first(Or something less expensive) to make sure everythings alright. I bought a lot of kits on ebay coming from different places all over asia and never had problems with it at all. Plus,shipping usually is faster than ordering from your local country.Maximum 3 weeks is what I had to wait to receive something from there.Prices are usually low and service is good so if I was you, Id give it a try especially for these very nice kits .
Give us some news and sorry for my "not so good" english!