As far as used scopes go, I'd try to find one of the older (early 80's maybe) Tektronix ones. These will offer better quality / more features than a newly made model for anywhere near the same price.
It's good to keep a lot of parts on hand as well, so you don't have to order them for each project. Resistors (1/4 or 1/2 watt), capacitors (all types), BJTs, MOSFETs, JFETs, transformers, diodes, bridge rectifiers, op amps, comparators, timers (555), PLLs, logic gates, flip flops, DACs, counter chips, digital comparators, voltage regulator chips (LM317, LM7805 especially), etc. are all good things to have on hand. You'll probably want to start with a large number of these parts, as ordering stuff for each little project gets old really fast (not to mention the shipping costs).
Other than that, I'd recommend:
-Function Generator. You will use it constantly.
-9 volt batteries and the connectors for them. These are wonderful for testing circuits, as they don't put out enough power to really fry anything (provided that you disconnect power as soon as you notice the circuit doesn't work).
-At least one big, fat power transformer. The one I always seem to be using is rated at 12.6V, 25A, and was $20 at
https://www.allelectronics.com (they don't have this one anymore, but there is a 5-amp one that would probably do fine).
-A big box of junk to scrap for parts. Computer monitors and power supplies are full of great stuff, and people throw them out constantly. This is a good source for heat sinks, transformer cores, large electrolytic and mylar caps, power resistors, large diodes, fans, and other more expensive parts.
-Some way to organize the parts. I would recommend the organizers that have a million little drawers.
-The pre-printed circuit boards that have breadboard-like patterns on them. These are WAY easier to build on than the ones with just a little copper ring around each hole.