Lots of questions to answer! haha. Well first off... the amps that were used early on in the Metallica days were Marshall JCM 800s. They're high end tube amps. They use Mesa Boogie Tube amps now... which are basically as good as it gets.
The difference between a combo and a stack is simply that when you by a head you just get the electronics portion of the amp (the amp part of it) you have to buy a seperate cabinet to put the sound through. This allows you to run more wattage. When you buy a combo you get the amp and the speakers all in one hence the name, combo.
As for tube or solid state? Well, you say you want to spend no more than 500 dollars on an amp... that's not going to get you a new tube amp. You might be able to look into a used tube amp on ebay or something. Sure, you don't want to get caught up in playing the name game with gear, but simply put... names say a lot. Marshall has been making amps ever since there were amps. They've seen Hendrix, Zeppelin, Pink Floyd, Metallica, Sabbath... and so on. The extra cost that you pay isn't for the name... iit's because you have to pay the freight to ship it across the ocean from Europe. Lesser known brands use lesser quality circuitry and components. But sometimes you can come across a high quality amp that will stand right next to the best of them. I think Burnout has probably played a lesser model from Marshall when they compared Behringer to Marshall.
I would recommend looking for a used JCM 1x12 combo. they get around 50-60 watts (which in the tube vs solid state transition equals 100-120 watts.). That's more than you need. That will blow your house down... haha.
Next... the guitar. Kirk uses ESP guitars with EMG 81s in the neck and bridge. I would suggest looking at the ESP guitars with active pickups. If you can't find one in your price range check out Jacksons, Schecters, and Ibanez's... they all make pretty good guitars. The stock pickups on any of the guitars by those manufacturers should give you a good 'Tallica tone.
Kirk also does use a Tube Screamer... in fact... he uses two of them. You should only need one. You don't need to get a Tube Screamer though... any overdrive pedal should work pretty well.
After all this... I am going to totally contradict myself and say... Buy a multi effects pedal board. A GENX by DigiTech or GT-8 (or 6) by Boss, or a PODxt Live by Line 6 would give you more bang for your buck. You can get a not so expensive amp and use the amp modelers to give you the JCM tone... they will have all the overdrive pedals and distortions you need in them... as well as a bunch of other neat gizmos to play with.
.....I think that's it?
