First I think that you should try to tune the Force engine or maybe get some help to do it. Some of these engines are a real pain to get tuned correctly.

Whether you get an OS RG or the TTR .21 engine, you need a starterbox.

I don't think that it is necessary to pay Tower's price for the TTR since you can get it on ebay for 100$ at the moment.

I have the OS RG in my LX Comp at the moment and it is a good engine and really easy to tune, but when it is worn out, I have a TTR engine ready to go into it.