Is that budget just for the integrated, or the whole system? I would say that you may want to budget for a good DAC, since you can definitely improve over the Slim's DAC. One route may be to get a DAC with volume control like the Bel Canto e.One DAC3 and then get an amp and use the DAC as a preamp. I know someone who does this with a Pass 250.5 and likes it very much (although maybe still misses a preamp).
I have the Pass and love it. I don't know what they go for used, but I would guess over 3,000. I would think a YBA Passion Integre 300 would be a good route, or the Plinius 9200, both of which are very beautiful sounding amps, and both of which hold up well for resale (also match your criteria above).
I would maybe suggest pinning down your speakers first, because if you're going to buy and sell until you get the right thing, speakers are a drag to do that with; however, amps are pretty easy. You could also get a 2000 and under integreated and get a DAC that will perform better than the squeeze, unless you're going with a modded unit.