Going to take the plunge and convert my XB8EC

[Patrick]

The first 3 items look good, and choosing a neu motor is also good, but the 1.5D's are a bit too high kv to use on 5s, especially if you live in a hot area.
I only have a truggy so I don't know exactly what would be better between the 1512 and 1515 for a buggy. Usually people with buggys use 1512's and people with truggys use 1515's, but that's not a set rule or anything.
The motor I have in my truggy is a 1515 2.5D (1700kv) on 5s, it works well and stay pretty cool. If you choose a 1515 for 5s I think the 2.5D is the best wind to choose.
Usually aim for 30 000 - 35 000rpm for racing to have a good balance of power and reliability. Neu's should still stay cool even if they're doing around 40 000rpm as long as it's geared right, but I'd still try and stay in the mid 30's. kv x volatage = rpm
eg 1700kv x 18.5v = 31450rpm That's only an estimate. Voltage drop of the batteries and how loaded up the motor is will affect what rpm it reaches, but it gives you an idea.
So if you choose a 1512 I'd look at the 3D (1700kv). The 1.5y (1900kv) or maybe the 2.5D (2000kv).

Oh, and welcome to the forum. I don't know why your post didn't get answer a bit sooner, that doesn't happen very often. Hopefully one of the buggy guys will be able to help you decide between the 1512 and 1515.