Aluminum siding came out in the early 60's (very expensive) and was hot in the 70's as prices became more affordable. It was sold as the ultimate siding: No painting and No maintenance. Right.
Nowadays, no one (that I have ever encountered) is applying aluminum siding and it is being removed from old houses at lightning speed. The main reason for this is that 20 years later, the siding is dented, faded and looks tacky....and.....newer paints, when put on properly prepared wood surfaces, will last (depending on weather conditions) two to three times as long as the paints of 20 years ago.
How your original siding looks will not be known until you pull off the old aluminum siding. From what I have read, many are finding the original siding in reasonably good shape, but of course this depends on how it looked originally. But it seems that aluminum did protect the underlying wood....although I only know what I have read.
The removal process is fairly straightforward, and is something any DIY homeowner should be able to easily do once you've taken off a few pieces and see how it is held on. A hammer, pair of plyers, flat bar and good gloves is about all you should need. Remember, the aluminum can be sold at a metal scrap yard.
Once exposed, poke the old wood siding along the bottom course, looking for any rot. Look for wood splitting or buckling. If you need repairs, it may be good to call a local established siding contractor (who does not sell siding, but just installs it) for an appraisal. Once sound, the wood is ready for scraping, priming and painting and looking original once more :-)