The plastic radiators crack over time. The one in my red XR2 just started cracking near the filler neck, I have no head gasket issues. These things see 15+ PSi at high temps for many, many hours. Plastic was a cheap "get it past the warranty" solution.
At 100k it's not time for a head gasket. They don't just 'go' in the 1.6 unless it's been overheated. Maybe time for a compression check if it's suspected, but don't go replacing them willy-nilly!
At 100k it's just starting to get broken in
I'd start with the radiator, get the all-metal one like has been recommended. They're not cheap, but you won't ever have an issue with it again. Replace all the coolant hoses, t-stat, etc while you're in there.
The valvecover gaskets on these cars like to leak, and it makes a mess. It's an easy fix. Also look to replace the distributor o-ring. Beyond that you have the cam seals (usually not an issue) and then front main and rear main seals. The front main can be done if/when you do your timing belt & water pump. The rear main is a bear to get to, do that if/when you replace the clutch.
The rack boots aren't a massive deal, they're probably a bit of a trick to install.
Watch the Ford dealers with these cars! They're Mazda mechanicals, and quite dated compared to what their mechanics are used to working on. Full disclosure: I have a healthy distrust for any dealerships. Don't let them scare you!