My 2 cents is unless that baler is real cheap then I would pass. I have heard that the Oliver balers were okay balers, but there was never very many of them around. Where will you find parts? Some the Oliver Hay line was bought buy Hesston back in the 1970's and was the foundation for the early Hesston hay tools line. Hesston is now owned by AGCO. I doubt AGCO still offers hardly anything for that Oliver baler and used salvaged ones will be few and far between when searching for used parts.
Best to stick with New Holland first or John Deere 2nd in vintage old balers. Those 2 had the market cornered and for good reason. Lots of new parts still available, used parts sources very common, and people know how to make them work.
Have fun on your 2.5 acre hay adventures, but take it from someone who bales only 4.5 to 5.5 acres every year. You should not expect to make any money. You will do good to break even at best (assuming you enjoy donating all your time for free and tinkering on old junk). Actually on 2.5 acres I expect you will certainly lose money.