Which PDP?

For 1970s PDPs, the easiest to aquire would be something in a PDP-11 or PDP-8. Since you want a unix-like OS with a C compiler, that pretty much pushes out the -8. 11/34s are among the cheapest PDP-11s from the 1970s (presuming you don't get a haul-it-away deal). The CPU, memory, and some peripherals fits in one BA-11 cabinet (10.5" tall, 19" wide, rack depth), but one can add a second BA-11 if one really needs more than 3 backplanes. The max memory is 256K (18-bit addressing), which is enough for something like a v7 or a XINU, but would be a bit cramped for 2.9BSD. Disks are probably going to be your biggest problem... the RK05 was a common disk in the 11/34 era, at 2.5MB per drive (4 drives per controller). v7 might fit, but 2.9BSD will be _really_ cramped. RL02s may be easier to locate, but they are 1980s tech essentially, and even at 10MB, are a bit on the small side for UNIXy work (but are great for RT-11, etc.) I never did solve my Unibus disk issue for non-MSCP disks... UDA50s are not impossible to find, but the disks tend towards the heavy side compared to, say, ESDI disks. It matters less to me than it used to, since it's possible now to use MSCP disks with 2.9BSD (formerly, one had to go to 2.11, and that required a split I&D machine, of which I have few). If an 11/23 is not too new (it might be 1979), it's a nice choice because a) it's 22-bit (except for rev A boards)allowing up to 4Mb of RAM, b) it _is_ compact, c) it's expensive, but a Qbus SCSI card is easier to find than a Unibus SCSI card. They are also somewhat common (I got my first one in 1986 for a whopping $300, then used that to do work on a contact that was my bread and butter for two years) An 11/23 w/RLV11 (requires CD-capable backplane) or RLV21 and a couple of RL02s is a nice PDP-11 system, and fits in a 42" tall rack. Not sure you could get all of that for under a grand these days, but it's a place to start.