I've two Plus 4's - a 1968 4 seater and a 1955 4 seat Drop Head Coupe so I speak from personal experience rather than regurgitation of often incorrect articles which have appeared in print over the years
The Moss box is a fairly agricultural device but capable of tolerating a great deal of abuse. Lack of synchro on first gear takes a bit of getting used to. Billy Bellinger has some mods for the Moss box which have been race proved.
Be wary of a worn clutch operating sleeve in the Elektron bell housing - an engine out job and a £600 machine shop bill to put right.
Repair sections available for the body frame so DIY repair is feasible.
Steer clear of Drop Head Coupes as a first restoration project - body frames and panelling are very expensive - budget £12,000 for a panelled Coupe body frame.
The TR engine is very robust but is on the heavy side! There are a few mods you can do such as improvements to oil sealing etc. Parts are plentiful and cheap...ish
For a full engine rebuild budget around £1500 for parts and maybe £650 for machining.
Chassis were not terribly well rust proofed from new so expect chassis rot! A car which has had a new chassis is a bonus here. Check the crosshead for cracks where the bottom tube passes through the sheet metal pressing. If cracked, budget about £1250 for a new cross head.
Two types of chassis - trunnion from 1952 to 1955 and then a shackle chassis from 1955 to 1969.
Hope this helps
Arwyn