Don't quote me on this,
but, I believe that the 3.3 is a direct replacement for the 2.5 engine.. I am not sure if you would have trouble installing this though...I can definitly find out by wednesday and repost if you do not have an answer. The conversion kit that you find is probably for converting the .15 engine to a .18(2.5r) engine.. I all may depend on what rusty you have a .15 or the 2.5r to decide weather you need a conversion kit..
And yes you can buy just the engine. The 3.3 engine runs great!....When its tuned correctly.. From my expierences they have a VERY small window of when they are tuned correct. They seem very finicky. I was also told to run nothing buy Traxxas Top Fuel 33% in the 3.3 or you could run the chance of overheating? LHS owner wouldn't tell me this for no reason.. so. The 2.5R on the otherhand will run very well, and is much easier to tune.. By this I mean that it seems to run good, even if it is not tuned precisely.
You also may need to add weight to the front and a pro-ball diff if you want to keep the front end connected with the ground, and pointing the direction you want.. It's going to be wheelie happy
Hope some of this helps