While I love my PCPs, I suggest you look at everything that is required to use one before considering what airgun to purchase. For instance, you have a scuba tank but is it a 3000 psi or 4500 psi and when is it due for testing? Hopefully, you didn't waste your money if it is due or overdue and a defect is found. If the tank is a 3000, you will not get the same amount of full fills for a 3000 psi gun (ie, the Marauder) as the 4500 psi will give so that may influence what you get. Also, did you find a spot that can fill it and do you need any special connections to fill/use it?
Aside from tanks, the next item is the air pump. As has been previously stated, these are great at times (tank is out of air, left tank at home, etc.) and a PIA at others (especially when you're looking at doing a good amount of shooting and you have do 70-100 pumps every 16 shots to "recharge" the gun). However, I feel that it is a necessity because when you have a tank problem it still allows you too shoot.
As for the airgun, make a list of all the animals you plan to use it on and then make a distance list for expected shots. You're choice of pellet may be influenced not only by your airgun, but also by the distance. You can check Chairgun for general ballistic data so you can see approximately what FPE you need at what distance to dispatch the animal in question. If you're going off of the "charts" for how much FPE is required to dispatch an animal, I recommend at least doubling that amount to help limit an issue you have. One of the problems I've had, especially with chipmunks, is using an airgun that is too powerful. When I'm using diabolo pellets (as I need the accuracy they have for 50 - 60 yard shots) out of a 32 fpe airgun for a body shot, the pellets to completely penetrate the vitals and exit the body, ricochet to who knows where, and allow the chipmunk to run off into a hole so it cannot be recovered. Even through the range is much closer (25 yards) when using Polymags out of a 15-20 fpe airgun, I've not had this happen and instead watch the chipmunks drop on the spot.
With the budget you've got posted, you need to account for optics, pellets, airgun, and possibly filling. So my first thought for you based on the scuba tank is to invest in another tank and get a gas-piston combo until you have the funds for a PCP (you can never have enough airguns anyway, right...). If you prefer to go with the PCP anyway (and there is nothing wrong with that) look at the Marauder Pistol (P-Rod) if you're shots will be limited to 30 yards and under. For raccoon, I'd recommend the Air Arms diabolo field heavy pellets (this may shorten your range to 20 yards), but for everything rabbit/squirrel size and smaller, the Polymags work very well.
