Well, for 1/12th scale pan car usually you'll need an ESC that works with 1S batteries while 1/10th scale buggy is standard 2S. Make sure both support a low turn motor (1/12 in open modified goes down to 3.5 turns on 1S) while buggies on 2S typically use 5.5-6.5T in modified on carpet and astroturf (usually a bit less power on clay/dirt). Hobbywing is an excellent choice for ESCs and their motors (especially in modified classes) are also not bad. For a real speed experience make sure to use variable timing in the ESC (boost & turbo settings). That's what makes a brushless system really come alive.

Servo for the 1/12 pan car I'd probably go for a Sanwa PGS-HR (or something similar) while for the 1/10th you should aim for 0.1s or quicker (60 degrees) and around 10-15kg force - lots of manufacturers available. For 2WD low profile servos seem to be more common by now, while 4WD buggies usually have a bit more room, you you'll find both, low profile and full size.