There's no one single forge that'll do it all.
If you get serious about forge work, damascus, etc, you'll end up with multiple forges.
I've got one blown vertical, a large ribbon burner forge, and a 2 brick forge with a t-rex style venturi burner.
If you just want to forge knives initially, I recommend buying or building a 2 brick forge. They're a great size for making small to medium sized knives and even larger knives, are very fuel efficient and with a good stand, take very little space. Buy a burner if you don't feel like building one. You can build the body and forge in a couple hours and just put the burner in.
You can't really forge weld in these though because the body is soft firebrick (and will get eaten by flux, and they don't operate as well at welding temps generally). You wouldn't really want to however. If you decide to get into forge welding/damscus/etc, you'll eventually want to build a vertical blown forge. They're easy to build and much cheaper to build than people are selling them for. They pretty much suck for hand forging blades though, since you can't "lay down" the blade in the forge you've always got to be using a bar of steel and forging the end, or attaching a handle of some type, which is always getting really really hot.
Personally I prefer to forge blades in a coke/coal (or better yet, charcoal) forge. They're cheap to run, and you can dampen them down and leave them running all day without wasting a lot of money on fuel, but they're a bit of a bigger investment in either space or effort in most cases. Although if you can find a good hand crank blower, a Tim Lively style charcoal forge is cheap to build and works really well. I started with one of these and still miss running it.
