You didn't say which one you got, the $350 or $1500 one. I assume the $350 engine which may be pretty cruded-up on the outside at least.
Before you install it, COMPLETELY CLEAN IT and see exactly WTF you have, every speck of dirt/oil...should be easy to get to (easy access) ALL parts of the engine to clean it well. That way, you can see if it has any leaks when you start running it. Of course, change the oil filter and oil...take the oil pan off -- also easily done with engine not installed yet -- and clean out any sludge...that'll tell you something if there's lots of sludge/metal particles or it's pretty clean. Make sure the various components'/covers's bolts/screws are snug (not overly tight)...you can reach everything easier before the engine goes into the engine bay.
Use anti-sieze compound on any hot areas prone to rust/corrosion, like the exhaust manifold studs/nuts, and exhaust pipe nuts.
If you want to do a through job, take the components off like the distributor, alternator, carb, and other things, clean each one, lube and reassemble. You can use kits for the carb. Be sure you don't mess up the timing -- if the engine has a distrubutor -- when you pull it out...note where the rotor is pointing and do not turn the engine while the distributor is out, so it can go back and be in proper time, not off a tooth or more!
When you pull the sparkplugs for the compression test, check each one...assuming they've been in there for a good while, they'll tell you what's going on in the combustion chamber. Replace plugs, wires if they look suspect...tune-up parts I'm talking about.
When you get it installed and you can turn it over (but not fire it up), you can do a compresssion check...find out what it's SUPPOSED to be in a good engine and compare it to your used one. All cylinders should be in the same ballpark within 10% or so max. Find out for sure.
Can't tell you about the rocker cover...get a Haynes Repair Manual for the car for starters...it will answer those equestions. You should already have one.
It comes with a radiator or it it your original one? You can use some white vinegar in the radiator if it looks like there's any white scale in it...probably water mineral deposists (use distilled or deionized water only after that...you have no idea what the other guy used...unless it's yours. If the radiator cap is old, it may not be up to specs re: pressure it holds...get another KNOWN good (new) one.
Yes, the water pump should be good, but at 150,000 miles (I assume) it probably should be replaced (maybe it is the original...they don't last forever)...it's easier to do with the engine out -- as are the serpentine/v-belts -- than in the car. And water pumps aren't TOO expensive. If it starts leaking later, you'll have to go back in there
-- probably have to take all the belts off -- again.
If it has a timing belt, do it also while the engine is out...it's a lot easier. Timing BELTS may look good but you can;t tell by looking...they're supposed to be replaced at certain intervals...they can go at any time and if they do -- and your engine isn't a non-interference engine -- you have big expensive problems, bent valves for starters. Again, however, don't take the belt off and then turn the engine or you'll mess up the timing...that is also in a service/repair manual like Haynes or Chilton. If it has a timing CHAIN, 150,000 miles is still a lot...find out when they are supposed to be changed. You may get by for another 50K or more but the teeth on the gears will probably be worn to points -- impacting accurate non-fluctuating timing -- and the chain tensioner may be shot anyway.
There are other things, but that should keep you busy for a while. Mostly common-sense stuff really. Just take your time if you can and do it right.
Good luck,
John D.