You can buy a nice TZ for $5k. If the playfield is dirty it's an indication the owner doesn't take care of the machine so other more serious flaws may also be present. Are the ramps or subways broken? Those could be in the $100 range to replace. Alkaline damage on the board? Another couple hundred. Powerfield mini playfield broken? Another couple hundred. Cabinet art faded? New decals $2-300.
Lamps not working worst case is alkaline damage on the board but much more likely to be a simple fix. Tearing everything down for cleaning will take many hours, how much do you value your time?
I think $3500 range is fair for all the risk you're taking. Lots of unknowns. Shopping a neglected TZ will take you more than a weekend if you're a first timer.
P.S. bigd, what is with your lack of vowels? Are they in short supply where you are? I notice you often leave them out of should, could, would and was just curious.