B4 is def still competitive. In the right hands, essentially as quick as anything out there.
6.5t in ANY car on polished floor is gonna be a real handful!
6.5 in any 2wd car on any surface is actually going to be a handful, and especially if your new to the sport!
My advice would be stick a 10.5 in it! You'll find that only the top top guys might consider a 6.5 in 2wd (and to be honest I'm couldn't say for certain they even go that far!). 7.5's are much more popular in 2wd with good racers, and at club level, most people will get much better results with a 10.5. A little less pokey, much easier to drive, and consistency and not crashing over 5 mins is actually the thing that will make you faster.
Straight line speed is not the fast way around in offroad racing!
As an idea, I run an 8.5 in an x6 at nationals, and am not being left for dead anywhere!
Sensorless, although not the optimum top level racing kit, is more than adequate for club racing. Proper gearing and on that esc, proper setup, will help with cogging. Check how it's programmed currently, see if it's adjusted right for you and your track.
Get a good baseline setup, then your thumbs will be the biggest factor...so practice
