If you already have a fat PSP, there are some dealers who will take in and give you a PSP Slim with a little top up of cash from you. Just go Google it and you will see the threads around.
If you haven't got any PSP yet, Slim is indeed a good choice. Its seriously very light and very slim. I have got the fat last time and did a comparison. Slim are now very cheap. Only the console itself cost you $280. But normally, people like to buy in BUNDLE form. This is because they are cheaper. But market price are: PSP Slim Standard pack, screen protector and 4gb memory stick for around $360 and modified firmware(aka version) to play download games(aka ISO).
Things to watch out for are refurbished sets. Some people use the parts of an old set and just put in a new casing and say its new. Remember to check out the "warranty seal". This is because if the warranty seal is still in place, means the PSP has not have opened up at all. This is also to ensure they dont change any parts of the set. Another common scam on the market is that some dealers provide FAKE MEMORY STICKS, which looks exactly like the real ones! There is no way which I known of to identify it right on the spot its fake. But there is a 95% of the fake memory sticks that lags your game! 95% is very very high! Because the actual pricing of a bundle in original dealers are around $400+. This is because they provide quality screen protectors and ORIGINAL memory stick with warranty! The fake memory sticks are much cheaper but 95% of them will lag the game you are playing. I have friends who got the fake memory sticks and they threw it on the floor and jump over it.
Actually, you can try asking the dealer before you make a purchase. Ask the dealer if the memory stick is MAGICGATE supported. This is because only ORIGINAL memory sticks are MAGICGATE supported. Thats all I can tell you.
Only honest dealers will tell you whether their memory sticks are original or fake. The only way is to find out yourself. Make them give you a guarantee. Try something like this:
You: erm, the memory stick will lag in games or not?
Dealer: wont la, i have a few customers with no problems.
You: you can say anything you want and who knows whether its true.
Dealer: ok, if the memory stick lags the game, 1 to 1 exchange.
See, like that they already gave your guarantee that it wont lag and if it does, you do have right to ask for an exchange. Be kind when you ask though, you dont want to get beaten up
Colour wise, I would recommend all to try to avoid Piano Black as the casing will show all your finger prints around due to its glossy casing.