If you are retaining the stock rims then go with 205/60/15 size tires front and rear. If you don't mind spending a bit extra you can pick up some 16x7.5 wheels (ask Luke or do a search for correct offset) and put some 225/50/16 size tires on there. With the proper offset and that size tire/rim you shouldn't rub but it will be a "full" fit. That size really does help the 400E deal with both its mass and power much better but 205/60/15 given a good tire is perfectly acceptable.

Hope this helps...Lee