How do I know if I will have enough power?
Is it the higher the KV the more power?
I am converting a Tmaxx & have a 3500KV rated motor. Will that be enough to get it moving, or do I need to get a lower KV rating?

No, higher kv is higher RPM which is speed since voltage x kv =rpm. A 5700kv motor on 2S will turn 8.4v x 5700rpm/v = 47,880rpm. A 7000kv on that same 2S would turn 58,800rpm. Higher rpm is higher speed.

You get more torque with lower kv motors - these can be geared up for good speed.

Tmaxx is heavy - what size 3500kv motor - 1/8th or 1/10th? 1/8th would be enough (maybe too fast depending on voltage you're running) and the 1/10th won't be enough for the weight - it may be able to get it moving, but its going to run hot.

That makes sense. The 3500 is off my 1/10th slash. Battery is a 2s 5200 7.4v. It sounds like I am going to have to find another motor.

A 3500kv motor can work, but it really depends on the physical size. A small motor won't have the performance you want without overheating unless you gear for ~5mph.

It's kinda backwards to get just any motor and then try to get it to work.

What should be done first is figure out what battery you want to use. Higher voltage is better for efficiency and the upper limit will be determined the highest voltage the ESC supports and by your wallet.

Once you know the voltage, you get a motor kv that matches it. Different motors have different rpm ranges. Feigao motors tend to do best at around 30k rpm max. Neu or similar motors can go much higher, but most stick to around 35-40k rpm. So, let's say you want to use 5s lipo and want to run a motor at 35krpm; the kv=35000/18.5, or around 1900kv.

Then, you pick the size motor for your application. Lighter/smaller vehicles can use smaller motors just fine. But for a T-Maxx convert, I'd stick with a larger motor, like a Castle 2200kv, Neu 1515, Feigao XL, etc.

You'd overheat that motor to the point you'd wind up with it demagentized and trashed. Sell it and put the bucks toward a MT capable motor. If you were planning on using the esc that goes with that, you'll find it wanting too. You need something that can handle the amps needed for accelerating a 10lb MT.

If it were me I would run it on 3 cell lipo with a smaller pinion. You can get some cheap 3 cell lipos and save yourself some coin.

Remember when you are calculating your total RPM to use 3.7v/cell with lipos. While they do have a max charge of 4.2v/cell when they are put under a load the voltage will drop. The nominal voltage is a good average to make your calculations with.

Also, the kv rating given on motors is unloaded, so that's what it will spin at when bench testing the motor not connected to any vehicle drivetrain. Once you put it in the vehicle the kv will drop a bit. Usually not big enough to be that noticable, but nonetheless it still occurs.