100K is not a lot for a Mk1. engines are as you say virtually bulletproof & some really good prices can be paid.
i presume you dont want to spend too much cos you are saving for the robbers who insure you.
look for rust in cills & wings. if the owner allows it, pour a bucket of water on the hood to look for leaks.
if a 5 looks good, sounds & feels right when driving, it usually is good enough to seriously consider.
good luck with the search.
Early 1990 1.6 Eunos classic red. modestly improved with Induction kit.
4:1 Stainless manifold & VERY LOUD stainless twin pipe system.
Black Racing rims (Watanabe lookalike). LOUD Klaxon as anti sheep device in Wales
Member of the RED Division, Living by the Bull & Bladder