Hi
I suggest you buy used in that case. I would look at the P30+ or Aptus 65, both should fall in your price range and you can always sell the H20 afterwards. An Aptus 22 is also an option if you prefer a larger sensor (and can find one), it should be cheaper as well, but starts at iso25 unlike the other two I mentioned. For trade-in options I'm sure the dealers will chime in. Also, try to get a back which will work with the adapter plate you already have, replacing it would be very expensive. Good luck!
Marko