Well, I take it you plan on racing this, or just bashing, or both???
That and price are the 2 biggest thing to consider in buying a mill.
Go with a quality .21 like a Novarossi P5, OS V-spec, WS7-II, and there are any others. These will be around $250-300.
OR you can get a .26-.32 mill. If you race I would stick with a .28 as the largest, as SOME track wont let you run a mill above this. Makers or .28s are STS,WASP,SH,HPI,Picco,Novarossi,RB,and ANY others. These can go for as little as $100ish(ebay) to $300.
Now comes the big debate..... .21 or .28????????
If your racing, your track and length of your mains will have something with which you pick.
Let us know about what you plan on doing with this(race/bash?) and if racing how big is the track?How much ya want to spend?
Dont forget you will need a clutch and a pipe for your new mill(engine), and you will need to factor that cost in.
I'm going to run a Picco .28(less than $200) with a Werks clutch setup($55) and a JP-2 pipe($40 used) in my truggy.