Its really not possible to say what velocity that AEG might put out, since all we know is that it is an M4. If you could get a model name from the seller, that would be much easier.
You should definatly try eBay for your camo and such, but do not trust their BBs. It does not cost a lot more to get some from BB Bastard and you know you are getting some of the best in the country. For the sake of your AEG and for the advantages in-game, buy from trusted suppliers.
Finally, don't always assume that these people are right in saying you need to spend $600 as a base price for your first gun. That is a lot to commit if you don't know how you will like your first game. My reccomendation is to try and borrow an AEG from a local Whitby player, try a game and see if you like it. I havn't met a single person who has not liked their first taste. Your first gun does not have to be a TM, or CA, or ICS. Echo 1 or JG make perfect starter AEGs that can perform admiribly and won't break the bank.
I personally reccomend either the Echo 1 G-36C, or the JG AK-47. Those are the only ones I have actually used. The quality of their internals aren't up to the standards of TM, but even if you replace the entire mechbox right off the bat, you have an upgraded AEG for the price of a stock TM.