For the money, the best DC I can think of is the
Harbor Freight 2 HP #97869, fitted with a
Wynn 35A cartridge filter (greater surface area, .5 micron filtration etc...), and fit a
Thien baffle in the inlet ring of that DC.
The DC frequently is on sale via a coupon for $139.00, the paper element Wynn 35A is $106.00 + S&H. I personally run the Spun Bond Poly but that filter is quite a bit more expensive at $150.00 + S&H. Even paying the extra premium for the spun bond, the total invested into this system is quite a bit below any other cartridge equipped DC on the market...
The Thien baffle is something you would have to make yourself. However I will tell you, after making several, they are easy to make, and WELL worth the effort in that they keep your DC filter clean and keep your air flow up.
I would typically suggest a separate cyclone separator unit with the Thien baffle, but it sounds like you are more concerned with space, than with ease of emptying your dust container...
Full on cyclones like what Paul and others suggest are great if you have the room, and the budget, but it sounds like both may be a concern for you...