See this is where I love stuff like the RPi's and other similar little boards. You can assemble a lot of processing power, stuff it in a lunch box, power it off a car battery ffs and jury-rig the whole thing together using crap bought from radioshack^H^H^H^H^H^H^H^H^H^HeBay (let's be honest), hell, people are building beowulf clusters out of them.
I suppose the key thing comes down to what you want the lab for as well, what are you learning?; basic linux server/client environment simulation? Pentesting? Networking? Or just hoarding tech? (still valid). For a lot of professional study a lot of value can be had with an 8 core AMD processor and 16-32'ish gig of ram and virtualise everything with either GNS3 for networking stuff or $hypervisorofchoice for regular PC environment stuff. Advantage being you don't need a separate room, don't need to kill your parents electricity bill, etc. etc.
Junk acquired servers are neato but the scream of rackmount server fans are a great way to make parents and housemates hate you.