I'd say a Crosman 1377, but it could be an issue for you to pump it.
I have a Crosman 2240 (.22) C02 7 inch barrel and it's NOISY ! maybe too noisy for the garden, unsuppressed.
I have a Crosman 2300T which is a .177 10 inch barrel C02 which is quiet(er) and very accurate, should be able to get one on your budget.
I tried someones Beeman pistol at my gun club and it was very quiet, recoilless and was cocked (compressed) with a single pump, about £70 too !
Whilst I haven't tried one, I'd say the Benjamin Trail NP (nitro piston) may also be worth considering.

Hope that helps !