1.4, 1.6 & 1.8 have the IB5 box, 2.0 have MTX75 box. Easiest way to tell is on the MTX75 box you will have a ring on your gearstick under the gearknob that you have to lift to get it into reverse, the IB5 is just spring loaded into reverse.
Even though your car is low mileage, the clutch could still have had a hard life, a cheap full kit you can find for around £80, its just peace of mind and could save you paying out for removal of the gearbox again within the next 50-80,000 miles, but obviously the decision is yours at the end of the day.
£70-100 for a gearbox on ebay, just be sure you find one for a 1.8 petrol as 1.8 diesel ones are different, any petrol 1.8 box from 1998-2004 will fit ok.
The voltage readings seem ok, so double check the battery connections are clean and tight. Could be a sticky starter motor also, had this a couple of times with my old Mk1 Focus - I took mine off, sprayed a little brake cleaner in it, left it an hour for it to evaporate then squirted a little WD40 in there, was fine after that.