Will standard gear ratios be ok if I stay with a less highly tuned engine? I am planning to use an all-alloy 80hp Suzuki 1300 initially, but upgrade it to a 130hp Suzuki 1800 later (largest capacity in the same engine family).

There is only set of ratios for the Suzuki gearbox (from the Carry/Jimny/Samurai) :
3.652
2.249
1.472
1.000
0.795

In comparison, the MX5 has :

3.136
1.888
1.330
1.000
0.814

IIRC, the purpose of a close ratio gearbox is to keep the engine in it's power band. If I keep a wide power band, rather than going for a traditional peaky performance engine, would there be any real advantage in a close ratio box?

How does the lighter weight of the Locost affect the choice of ratios? The lighter weight will give me better acceleration in all the gears - will this exagerate the wider spacing of the ratios or is it really not an issue?