The transmission is often the weak link in the A-Series even in standard form, but even more so when tuned. It is false economy not to rebuild the transmission when overhaulling the rest of the engine.
A basic gearbox rebuild is not to complex a job & most compotent DIY mechanics should have no real trouble. I would advise getting hold of one of the older Haynes manuals as this covers rebuilding the transmission in great detail.
The Metro Turbo has a four speed helical box as standard, with a 3.2 ratio final drive. The gearbox is identical to all other A+ gearboxes, apart from a larger first motion nose bearing. See here for more information.

The standard helical transmission, providing it's properly rebuilt is adequate for standard power outputs and can prove reliable even on mildly tuned motors. The first thing to fail usually is the diff, i would advise uprating this, even on a standard motor. The £150 pricetag for a crosspin diff is well worth it.
The next weakest area is the drop gears, usually ok for a standard engine, but any real increase in power will require the use of straight cut items.

This is a straight cut gearbox, essential for high output motors. Note the central oil pickup pipe fitted. This is a cheap and worthwhile modification to do while you rebuild your gearbox, & is essential for "spirited" driving. This helps stop oil starvation to the engine under hard cornering.

Check everything for signs of wear, & replace all bearings as a matter of course.
This layshaft had been fretting in the casing. Both the shaft and the casing had to be replaced.

One of the bearings the laygear run on. The layshaft will often show heavy signs of wear from these bearings, due to poor quality oil, infrequent oil changes or the use of cheap poorly hardened layshafts.

One of the two bearings the main shaft runs on. These bearings MUST be replaced, as these are usually the first to fail.

While the gearbox is out it is easy and well worthwhile tapping the case to install an oil temp gauge. See oil & oil coolers for more information.