I agree with Dave about the 5050..but there are a couple other units to consider...
the 2020 is a half rack with most of the same sounds as the 5050, including 2 slots for expansion..It has 40 insert effects compared with twice as many on the 5050...
Neither of these will give you the capability to play"Scat" sounds..the vocal expansion board will not work with the 5050 or 2020, and there is not a compatible SRX vocal board yet..

This brings me to my suggestion..get a JV1010, still has decent sounds[less the xv stereo sounds,but stellar enough], includes the famed"session" board , and a single slot to add a JV board[like the vocal board]..
You can buy a used JV1010 for under $200 and a used Vocal board for under $100...For under $300, you have a valued module..in a space saving half rack unit..