1) Lots of natural light, ideally a corner room with lots of windows. You'll also need at least one of those magnifying lamps.
2) Deep benches, at least forty inches, this is because your test equipment will take up at least a foot of space at the rear.
3) Lots and lots of mains sockets, you'll never have enough. Wire the power through a residual current circuit breaker and a big red emergency stop switch. Make sure your family and other people around know where that emergency switch is.
4) Four channel scope, signal generator, lab power supply (0-40V 5A) with a couple of channels, a second fixed power supply with 12V, 5V and 3.3V outputs and a bench multimeter. DON'T buy cheap, it's better to get a good second hand unit than a piece of cheap Far-East test gear. I like Hameg but I know that opinions will differ here.
5) Anti-static mat and wrist strap.
6) Lots and lots of storage for parts, as with mains sockets you'll never have enough storage.
7) Decent tools, as with the test equipment don't buy cheap. I'm still using some tools that I bought twenty years ago.
8) A set of drawers underneath your workbench for storing your tools. The plastic inserts that go inside kitchen drawers will help keep things in order.
9) A burglar alarm and a lock on your workshop door. All this lot is expensive and you don't want it to vanish and reappear on Ebay.
10) Air conditioning and/or heating depending on your location. Equipment calibration will drift in temperature extremes and the standard of your work will suffer.
Ganty