I think your starting point must be that it will not make you money, and at best it will cover your costs. So the bottom line must be that you want to have the cottage for itself and not as an income generator - obviously it may be a good capital investment, but that's long-term.
If that's OK, then I can give you one example of things done right & one not so.
Property A: neat, extremely clean, efficiently done-up small house in easily-reached Cornish seaside town. Typically-popular sort of size: 2 bedrooms (one double, one twin), will accommodate a couple more in sofa-beds in living-room (= convenient but also flexible = wide appeal). Agency NOT used, nor website, so far as I know: lettings done through workplace notice board, friends-of-friends, etc. Deposits: yes. Tax bits: don't know, sorry. Housekeeper: ESSENTIAL - absolutely sparklingly clean - done-up in the first place with this as first principle. Other good things: masses & masses of information provided; comparatively very reasonable price.
Property B: big, rambly, eccentric house on Cornish hilltop. Lots of bedrooms, outhouses, books, character, mismatched furniture, hit-or-miss heating etc., peace/quiet/remoteness (= narrow appeal). Agency used: lack of face-to-face lettings system = tenants vary from the ecstatic to the horrified. Tax bits, not sure (but I do know letting Property B does not cover its costs - so it's possibly more an allocation of losses ...). Housekeeper: not nearly as good as Property A - but, due to remoteness of property, options are limited, so make do with what there is (housekeeping failings contribute to certain tenants' horror, naturally). Because of the size, pretty expensive.
To extract a moral: a small, neat, not necessarily very characterful, well renovated, easily maintained, easily reached property, with access to excellent housekeeping support, is going to be a better investment than anything wild & wonderful. Which might conflict with what one dreams of in a cottage ...
Sorry for length of post!