The motor pictured has a 2 pc rear main seal (the flange on the crank is not round) so it's a MARK IV version which was produced until 91-92ish. The combo v-belt / multi groove pulley on the water pump would be from around '88+ so you can bank on it having peanut port heads which are fine for a bone stock motor but do not breath above 4k rpm so they are not good for performance applications. The long filler neck plus the drum brake on the trans would indicate this is probably from a motorhome. My guess is that it's a #14015445 block which has about a 50/50 chance of being 4 bolt main (these were produced both 2 and 4 bolt, the casting was common in later model truck, Sub and HD applications) but it'll have regular bottom end parts.
As stated, the trans is a TH475 which is basically a TH400 with some heavier internals. IT's not the Holy Grail but a nice part. I used one in my old K10, remove the brake and tail cone and the tailshaft is the correct length for a pre '85 / figure 8 adapter NP205 set up.
Is it worth it, probably. The motor is mostly complete so you'll get lots of hardware, covers, oil pan, balancer and hopefully the front end brackets, all of which adds a little value. I would buy it with the mindset it's a builder, not something to try and run as is since there's no history. The trans is a nice part to use or worth about $100+ to sell as a core.
All that said, it's Craigslist so it may be total junk. I'd try to get it down some to $650-$750 ($500-$600 would be awesome).