Welcome to the forums! And congrats on the truck!
Unless the Volvo is of similar displacement and compression, there will be a difference in the effort required to turn it over, but if you have the proper equipment, turning the engine over should be a piece of cake. As for the engine not turning over and the oil being somewhat low and black, those aren't very good signs. It sounds like there may be a bad sludge problem, the motor may have even locked up from oil starvation if the pump went out or something of that nature. Thats worst case scenario though. take off your passenger side valve cover(the easier of the two to remove), and see what it looks like under there as far as how the valvetrain looks.
Some other common tahoe issues:
1)Check the antifreeze for oil and vice versa, intake gasket leaks are very common in these trucks. The OE coolant was acidic and ate through the gaskets, it also reacted badly with the silicates in normal antifreeze, creating major issues.
2)Check the quality of the spark plugs, the color/condition of the plug can tell you a lot about your engine
3) make sure your starter is fully functional, I know its obvious, but hell it beats chasing down some mythical engine gremlins for days when in reality the motor is fine.
4)Once you finally do get the pistons pumping, do a compression test. I highly recommend it, a compression test can tell you a lot about the condition of the engine, dead cylinders, cracked heads, and bad gaskets. You will find that most of us on here resort to a compression test to sort out most engine issues that deal with the combustion chamber performance on an individual basis.
Good luck man, post back and let us know how it all works out.
---------- Post added at 09:00 PM ---------- Previous post was at 08:58 PM ----------
edit: and yes as 95blacktahoe said, an engine swap usnt that hard of a job, and will most likely serve you quite well in the long run.