No hard and fast rules on the gearbox mileage, you just have to use your ears. Our mk.1 has done over 170,000 miles (275,000KM) and the gearbox is still fine. Some have had failures around 75-100,000 miles.
All the L-series engines are pretty bomb proof. The cam drive chains are over-engineered for the job, I think you would be unlucky for one to fail, even if the engine had been neglected. Very rare to hear of water pumps failing, but the pump is external and driven off the auxiliary belt, so relatively easy to change.
As Mick says, the DSI engine (dual spark) plugs are bog standard plugs and are only a couple of pounds each here in the UK. The later VTEC engines use the iridium tipped plugs. Yes, they are more expensive, but there are only 4 and the replacement interval is much longer, so it's not the end of the world.
Here in the UK, we only had the 1.2/1.4 DSI engines in the mark 1. Not sure what the mark 1 1.5 Jazz/Fit would be, Honda did make DSI and VTEC versions of the L15 engine early on.
Seems the Vtec version is probably not the one to get, I saw someone on the fuel economy thread mentioning their 1.4 has lots of low down torque and looking at the torque specs the 1.4 gets 119 N⋅m (88 lb⋅ft) at 2800 rpm whereas the 1.5vtec is 145 N⋅m (107 lb⋅ft) / at 4,300 rpm, I would much prefer the torque at lower revs because one thing I don't enjoy about my CRV is the lack of lower down torque it has lots of power but you have to rev the hell out of it.
We have the 1.4 DSI in a 2008 mark 1, and a 1.4 VTEC in a 2015 mark 2. I actually prefer the DSI, it is far more tractable at low revs and is easier to drive around town. The VTEC is gutless at low revs, but the power is there when you open the throttle up.
I believe the 1.5 VTEC is a better proposition, but fuel economy suffers a bit.