Tire size question

[Booeyman]

Greetings:

I have a transformer and it has 4 lug hubs. I currently have a set of 15"x7(.5)"? front and 15" x 9.5(10?)" rear.
The tires on it are really nothing but rollers and all the wrong sizes (rear barely roll due to size).
What size tires would be proper for these wheels?

Also, is a 5 lug kit available to change over. I would like to get coffins for it at some point and don't care for the 4 lug version.

Thanks and regards;
Tony