I've a 2 stroke bias as well - although 4 strokes have the advantage at the pump, are somewhat quieter and a touch more torque, ya just can't beat a 2 stroke for simplicity, power to weight, size & fit, and best of all - price.
Honestly - it all depends on how much the motorhead ya are, a 4 stroke kit ya pretty much bolt it to the bike and (hopefully) forget about it - unless ya wanna make it look
groovy, in which case you've yer work cut out for ya as ya deal with trying ta make it look like something other than a lawnmower (the blasted recoil start mostly).
Whereas with the 2 stroke, for the same effort and money you avail yerself of the plethora of aftermarket performance parts and build yerself a screamer. It starts out lookin' like it's meant to be there - so the time is spent makin' it stronger... the poor 4 strokers haven't the selection of performance parts.
Not to say you can't
have an awesome lookin' 4 stroker - there's quite a few on this forum, but again - it's all about what you
want outa it
With the above choices, they really do end up costin' about the same and just about the same amount of "work" heh, unless ya leave 'em stock - in which case the 2 stroke costs less than half the price of a 4 *shrug*
General rule of thumb;
4 stroke = torque - pulling power
2 stroke = horsepower - speed
As for inframe vs rack mount... tho rack mounts are by far the simplest installation (usually), in frame "mid-engine" mounts have the advantage of better weight balance as they're lower and in the middle of the bike. While rack mounts are awesome too - there's a reason motorcycles have their engine where they do