I've purchased a house in which the previous owners have recently used
acrylic paint over oil paint. I say this because the acrylic paint can be
scraped off very easily (with a fingernail), and it appears to be oil
paint underneath. I would like to repaint with a latex. Do I need to
scrape off all the acrylic, or can I use a primer over everything, then
proceed with the new paint job? (If so, which primer?). Any other advise
would be appreciated.

No use just priming and painting, as this will have no effect on the bad
bond underneath, and your new paint will continue to flake off. Best
solution is the hardest - Use a wire brush, hand-held chisel, ice pick,
scraper, etc, and locate (there may be areas where the bond is ok) , then
scrape off loose acrylic paint using a combination of tools depending on
shape of trim or wall, then scrub and sand rough edges and exposed patches
of oil paint. Once this is roughed up, spot prime the exposed oil paint,
fill where necessary, reprime the dried filler, and repaint whole area using
acrylic or oil. I know it is a lot of effort, but likely the only way to
proceed to get a lasting paint job.