Just because the car runs badly I wouldn't assume it needs a rebuild. I don't know what your mechanic was thinking.
The things you should look for a signs of needing a rebuild
Low oil pressure - could be worn bearings
Blowing blue smoke - usually rings
Fuming badly from breather - usually worn rings (can be head gasket though)
If it's not doing any of these things then it unlikely it needs rebuilding
I'd be checking all the things that have been mentioned previously
Vacuum leak is a very common malady that makes a car run like a dog.
For checking the tune of the carb. Single SU's are very simple. Check this site
http://www.moreg.org.au/carburettors.htm
Keep us posted on your progress, I'm sure the combined knowledge of this forum should be able to get it on track.
Pete