Hey... glad to hear that there is another Xray fan out there. I just did an Xray XB8EC a few months ago and I love it. Huge upgrade from the CEN I was running, but the CEN is still the ultimate bulletproff buggy IMO (with the exception of the wing mount).
Anyway. Choosing between those 2 motors kinda depends on what you are going to be doing with it. For a basher and high speed run buggy go for the 1515/1Y its brutal fun power. For a track buggy I would go with the 1512/2.5d It will be more than enough power and give you longer run times.
As for your other questions...
1) I actually run the Hacker 10xl on 5s in my basher buggy and it runs smooth, powerfull, and cool. The Hackers are a giant upgrade to the Feigao based motors. I wouldnt quite put them into the class of a Neu or Lehner though, but for sure an excellent motor. If you can find one for a reasonable deal I say jump on it, but for full price Id spend the extra couple bucks and get a Neu or Lehner. You still need to run the RCM heatsink/motor clamp on the Hacker B50 series. Not because of the heat, but because the endbells can pop like a feigao.
2)yes you can run a buggy on an L can motor, but with those lower end motors you are better off with the XL for heat reasons IMO.
3) With the same KV and same voltage the L can would draw less power than an XL, but it will produce less power too so you could get faster speeds and better acceleration from the XL by gearing up.
4)Yes. Pick an efficient motor and gear it properly.
4) (yes 4 again) I'd skip the new LMT forced air mdel motors for a buggy. Itll just suck the dirt into the motor. Thoso are designed for airpalnes and Helis IMO. The standard can LMT will do you just fine.
A 1940/7 on 4s will easily run your buggy to 40mph. I runn a savage on that motor and can take a 12 lb savage over 40 on 4s with 20mph wheelie power to spare.
I have run my 1/8 buggy on a 1030/10 on 5s and it totally ripped. Its all a matter of gearing it properly. A 1940 should be planty of motor for a buggy and a 1950 is excess,but in a good way
Just get one of the Neus now that the MM appears to be able to run them better.