There are a few very good ships that are just out of your current price range (e.g. Python, FDL, Krait II), but a few others that are within your budget if you absolutely need to buy a combat ship right this instant. Personally, I would recommend waiting until you have around 150M CR and then buying yourself an FDL and equipping it as a shield tank.
That said, if you're looking to buy a combat ship on 90M CR and still have enough for a rebuy, I would recommend going with the FAS or Chieftain. Both ships are very similar to each other, with good maneuverability, OK-ish shields, and a thick hull. The Chieftain is a bit slower than the FAS, but is more maneuverable, has a stronger shield, and has more numerous hardpoints to allow you to experiment with different weapons. Both ships can be bought and outfitted for under 85M CR, which will give you military bulkheads, a boatload of HRPs and MRPs, the biggest bi-weave shield you can fit (you will lose your shield in these ships, you want it to come back as fast as possible), A-rated core internals (except D-rated sensors and life support), and your choice of weapons and utility mounts (provided that you don't go overboard and get all PAs, railguns and shield boosters). If you are concerned about possibly not having enough money for more than 1 rebuy, you can downgrade the bulkheads to reinforced and shave ~10M CR off of your purchase price and 500k CR off of your rebuy at the cost of less than 300 hull (you'll still have over 2200 hull without engineering).