I've used just about every MPC ever made and currently own the MPC-2000XL Studio Plus. They're great machines. Alot of the same functions can be done with software synths thougth. They even read the same formats (soft synths and the MPCs)
You can get an MPC-2000 Xl Studio Plus (which is the fully loaded unit) for $800 used. Soft synths and MPCs have their own sound so it depends on what you want sonically.
Reason 3.5 is also great. It does alot more than just sequencing and drum sounds. The MPC-2000XL is based on akai's older S-2000 sound engine which is limited to drums because of the way it's constructed. It has great string patches, rhodes, etc. It's really great and i use it for jingles and stuff like that. I prefer the MPC for R&B and Hip hop stuff.
i hope this helps
Keep in mind there are other MPCs you can get
MPC-60
MPC-60II
MPC-3000
MPC-3000LE
MPC-2000 discontinued
MPC-2000XL or MPC-2000XL Studio Plus (the expanded version)
MPC-2000XL Limited Edition (they came out in special colors. Black and Chrome, Orange, etc.
MPC-2000Xl ZP which adds a zip drive
Akai MPC-4000 which has the sound engine of the Akai Z8
Akai MPC-1000 which is a more affordable MPC. I don't know what the sound engine is based on.