I picked up a jet jmd-18pfn the other day. I'm not a machinists jut a guy who doesn't want to use a drill press for milling. while this is older ,made in 2005. I don't think it was used for milling at all when I checked for run-out it was .0002. the table is dead square to the spindle. at least as far as I can tell. my question is will I be able to use this to flatten the top of a piece of rr track and whats the largest size face mill I should get. its has a 2 hp motor 230 power. 

You can use that to cut RR track, If you have a long piece just mill the slides travel at a time and re-clamp. As far as what to use, just use an older mill bit you already have, or a face mill, or a fly cutter, or anything that fits in your collets. Buy the cutter that you get the best deal on, 2"-3" seem like a useful general purpose size to look for though. Keep an eye out for Facebook/craigslist/auction bulk lots of endmills for some really good deals. You'll probably find 1/2", 3/8", and 1/4" are really useful sizes in just about anything. That mill is a game changer for sure, wait untill you put a cheap digital readout on it though!