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The Teleskop-Express Photoline series has several scopes that would suit within your budget, running at f5.5, and using FPL-53 objectives. While native f7 they reduce using the 3" screw on Photoline Reducer Corrector to f5.5. A triplet, running the four-element reducer is a great combination. The Image Star range with in built correctors are more costly and you lose the option of using them visually should you wish to do so.
I have the 115mm Photoline Triplet APO running at f5.5 and it is used for shooting narrowband. If you want even wider field options, the TS80 Triplet APO is a bargain, it reduces (using the same 3" Photoline reducer corrector, down to f5.5 (440mm fl).
You could look at Hyperstar options, say with a Celestron 8" SCT, or Edge HD, running the front mount camera but you need to use a camera like the ASI1600 to avoid increases on obstruction. This config wpuld be more expensive than the TS Photolime APOs but it is much faster at f2, however you may need to buy high speed narrowband filters.
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