Type 9 Box Rebuild

I posted a link to this ebay item elsewhere but have since started to wonder if it might be worth picking up to rebuild myself (as a spare incase the good one gives trouble during an event but also to learn a new skill).
http://cgi.ebay.com.au/ws/eBayISAPI.dll ... TQ:AU:1123
I know nothing about gearbox internals. How feasible is it to rebuild one at home? Special tools needed? Does the rust on this one look too much? I am picking that gearboxes are not something to mess with and maybe something of a black art?
If it is all there as he says and the majority is useable (other than bearings and seals say) then might be a cheap 'insurance' spare?!
http://cgi.ebay.com.au/ws/eBayISAPI.dll ... TQ:AU:1123
I know nothing about gearbox internals. How feasible is it to rebuild one at home? Special tools needed? Does the rust on this one look too much? I am picking that gearboxes are not something to mess with and maybe something of a black art?
If it is all there as he says and the majority is useable (other than bearings and seals say) then might be a cheap 'insurance' spare?!