Even a stock rebuild would cost more than that, but to find out how real the "top to bottom" part is and to get an idea of what I am buying the questions I would ask are:
Is it a 2 barrel or 4 barrel (heads)?
What intake does it have?
Is it a stock rebuild?
What was done to the heads? (I.E. valve guides, valve springs, retainers, hardened seats, studs, roller rockers...)
What type pistons (forged, cast, ...)
Compression ratio (helps determine what type heads they have (2V will be lower compression unless they have pop up pistons or Aussie heads). 4v generally, but not always will have higher ratio. Bottom line--do they know what the heck they are talking about or was it thrown together/BS on a rebuild?
2 bolt or 4 bolt mains?
High volume or standard oil pump?
Oil restrictors installed?
What cam was installed?
Are accessory drives included? (Pulleys, PS pump, water pump pulley, alternator brackets)
Installation challenges will vary, but you will need new exhaust manifolds/headers, will need to get your exhaust modified. You will need the accessory drive stuff mentioned above as your 302 stuff won't fit. The C4 may need freshening depending on the build of the Cleveland.