Original NES systems can be tough to buy, but you have plenty of options. You may find a great deal, but end up with a lousy 72-pin connector inside and spend all your time jiggling cartridges trying to get the stinkin' thing to play. Some people sell refurbished ones with new connectors inside which will run you more. You can also buy a new connector and install it yourself if you find a "dead" NES at a garage sale or something. They go for around $8 on eBay and are fairly easy to replace. Here are great instructions:
http://www.jandar.net/nes72pin
I've owned two NES consoles in my life. The first was a gift in 1990, which worked reliably until last year. It's still sitting around here somewhere. The second one I found by luck at a thrift store for $25 with a bunch of games, barely used and works great.
eBay has many systems that range from super-cheap (prob. broken systems) to extremely expensive collector's items. It seems the average price for a refurbished NES that will work, with a controller or two is ~$50. Do some hunting and decide what it's worth to you! It's my favorite system so I have a tough time naming my max price on it, even as big of a CAG I am.
You could also get something like an FC Twin, which plays NES games but is not an NES. I have no experience with these, but here's more info:
http://en.wikipedia.org/wiki/FC_Twin. The NES-101, often called the top-loader, is another option with a more reliable cartridge connection, but they run a bit more out of rarity.
As far as good games, here are some of my favorites:
BurgerTime
Toobin'
Bionic Commando
Bubble Bobble
Double Dribble
Excitebike
Marble Madness
Super Spike V'Ball
Punch-Out!!
SMB 3
Rad Racer
Also check out
http://www.nesguide.com which archives gameplay videos of NES games on YouTube. This may help your search for games you'll like quite a bit.
Oh, and give the CAG trading forum a look. Occasionally someone will put an NES bundle up for sale.
