You have a servo to run the rudder? I have a spare hull that I think I'm going to get rid of. All you would need is a brushless motor and esc from United hobbies which would depend on how fast you want to go. Half horse power(ah say 400 watts) should be good enough for the 25" hull that I have, I would go with something in the range of 1800-2300 rpm on 14.5v, so that would put the amps in the range of 30-40 amps, so we'll call it 40 amps.

Pretty much most of the guys on here go surface drive, if your handy you can make your own, or the extremely cheap route is to go subsurface drive which isn't as fast but it is fast out of the whole and turns on a dime compared to surface drive. Here's a video of my subsurface hammer on brushless, with trim tabs unadjusted(one was digging in too much).
http://uk.youtube.com/watch?v=U63hLBHb3zg


So parts wise...
Hull 90 bucks
motor 40-50 bucks (http://www.unitedhobbies.com/UNITEDH...4&ParentCat=59)
Esc 30-40 bucks (http://www.unitedhobbies.com/UNITEDH...&ParentCat=182)
hardware (http://www.offshoreelectrics.com/categories.php?cat=17)
15-30 bucks subsurface
90-120 surface with flex line

Price wise cheapest route you would be looking somewhere around 200 bucks and it climbs from there.

It really depends on what you want, I personally want to build my own boat rather than buy an sv27 just because it would be more fun and I want it built better with a flooding chamber (so I don't have to wait for wind to blow boat to shore when they flip over, which is guaranteed). But is also up to your skills.

You could also buy a used one online, or get some sort of ebay toy that has been a good candidate for swapping over to brushless although some of the hardcore guys on here would suggest otherwise, but it's still a viable option.

Ivan.