I'd go with micro four thirds for lens choices. A used epl1 is under $100 (~90 at KEH usually). The sensor sucks for low light (ISO 1600 in emergencies only really), and there's little headroom in the RAW for highlights and shadows, but that should not matter.
For any serious macro, you need a flash. A $40 yongyuo is quite capable, and eliminates the need for a more expensive sensor. Live view focusing is dead on accurate. Weak AA filter helps here too.
There are several options for lenses. How much magnification do you really need?
A regular cheap 50mm f4 macro (Canon FD, Nikon F, m42, Pentax K, Olympus OM, Konica etc.) Would be good enough to see most circuits.
If you want more magnification, a bellows, is a cheap way to get more magnification. Enlarger lenses are also very very good for their price.
E.g. this is $25 shipped.
http://m.ebay.com/itm/like/261696205087?lpid=82
It will deliver excellent results on an m39 bellows. This one just happens to be m39.
Get an m42 bellows (can be found cheap) and an m42-m39 ring.
http://m.ebay.com/itm/261698413019?nav=SEARCH
I got several bellows for $20 and under. Just have to look around.
http://m.ebay.com/itm/300826431034?nav=SEARCH
M42-m39 adapter is cheap.
Epl1 (100) + lens(25) + flash (40) + bellows/adapter(40 - assuming they run high)
You're under $200
The zoomed in component was with the Enlarger. The other was with a Vivitar 100mm macro, which does 1:1 without adapters). Note, enlarger lenses don't have a focusing helicoid. You focus moving the lens towards the camera or away from it on the bellows. Rather, you end up setting the magnification the position gives you, and focus moving the full camera in practice. I don't usually use a tripod. No need with flash, though your budget allows for it.