As a general guide the cost to carry out a full strip and re-build on a S1/2/3 Lambretta ( excl. TV175 S1 ), which is complete and in a fair solid condition is around £7500.
This approximately splits into ;
Body strip/prep/paint £1500
General Body/Engine build Parts £1500
Labour £5250. ( 150 hours @ £35/hr )
This is to completely strip the scooter to the last nut and bolt - complete paint removal to the bare metal and then re-build to modern standards using 2-pack paints - in 2 colours, 12v Electronic ignition and lighting, new cables, speedo, wiring, rubbers, badges, shockers, tyres/tubes, brakes, re-bore + new piston, bearings/seals, crankshaft, exhaust, etc, etc....
Obviously any upgrades that you wish can be incorporated into the build - with only the "upgrade " cost to pay if instructed before that particular item is built to standard specifications.