Its all dependant of what you want to do with it. If you are gonna be racing, you need to stick with a .28 or less. If you were gonna strictly bash, I'd say go with a .32. Any of the engines mentioned in this thread other than the novarossi's and OS's are under or around $200.

As far as total build cost of an LST, you can spend anywhere from $5-600 to $3500, even more easily. All depends on how deep your pockets are. LOL.