Penance KH and GH are much more meaningful (for coffee making - both taste and ‘maintenance’) than TDS. This said, if you have highly mineralised water (as you seem to), definitely don’t use tap water.
The kit seems fine; for coffee making purposes, the ‘resolution’ given by the dose indicated is usually not adequate (17-18 mg/l CaCO3 equivalents per drop) - just use 4 or 5 times the amount of water (therefore requiring 4-5 times the amount of testing fluid for the same GH/KH result).
Osmio filter - to some extent; I don’t think there is a guideline, though (as in “if you have x KH, y GH and z TDS your filters will last for nnn litres”).
Bottled water - Ashbeck and Lockhills are popular; neither is ‘ideal’, and a mix of the two is probably a better balance. Personally, I’d rather buy distilled/demineralised water and add a bit of bicarbonate until you get the Osmio.