AD&D 1e is the one true system. Everything else is a supplement. Even Hackmaster.
That sentiment aside, I'll tell you about Hackmaster. Hackmaster is a brutal game based off of 2e AD&D, with 1e and even some older rules thrown in. It also is easily interchangeable with AD&D, like C&C is. It is a stand alone game in it's own right, however, and is comedic in flavor. Much of the comedy is aimed towards grognards, though there are some amusing parodies of 3e as well. The game can easily lose all of the comedy and still be a stand alone system which will be a slightly beefed up skeleton of 2e/1e AD&D.
The game is nice because in addition to being a stand alone game it can be used as an Unearthed Arcana-style supplement for AD&D. The price of the GMG and PHB are no more outrageous than many of the things currently for sale, though the MM's were badly compiled. You have to buy eight books to have the entire bestiary. A far cheaper road to travel would be to buy the 2e Monstrous Comependium or the 1e monster books (MM, MM2, FF) and add twenty hit point kickers (or two hit die) to each monster. The modules are mostly comedic homages to classic AD&D and B/XD&D modules from the good old days. A notable module is Slaughterhouse Indigo, very well written. The core rule books are paperback. The artwork is very funny, and if you are familiar with AD&D you pretty much already know how to play Hackmaster. No new material for Hackmaster has been released in a long time, causing many folks to move away from the game. Thing is, you don't need any more material. The core rule books are good to go, and whichever monster book i mentioned you choose to go with (1e, 2e, or Hacklopedia of Beasts).
Moving from 3e to HM will be a big shock (assuming you haven't played AD&D before). There are no set mechanics to limit the DM, as
A. Such a thing is not old school in spirit, and that is what HM is all about
B. The DM is expected to use common sense
A character's life expectancy is usually not long unless skillful playing and dumb luck come into the picture, or unless you eschew the quirks/flaws. Combat is brutal in Hackmaster, and spells aren't quite as tough. The game is built largely around combat, but the amount of roleplaying in any game system is dependant upon the players and DM, not the system. I have had a blast with the Hackmaster products i have bought, as I have mostly converted them for use in my AD&D game. If you're the type who becomes too attached to you character, and doesn't like alot of comedy in his games, I wouldn't even bother buying this game. Stick with Vampire: The Brooding. But if you are out for a good time with lots of mayhem and laughs, Hackmaster is for you. A game that can be played seriously or with lots of silliness. From Ridley Scott to Monty Python.
Your call.
P.S. you'll find some dorks who take this game way too seriously, like the afore-mentioned "manly" Hackmaster players. Don't let that deter you, you find them in every gaming circle.
There are 3-5 adventures and one supplement being worked on right now, but we depend heavily on freelancers as well, and these things take time.
