Pat

That is a completely reasonable strategy and it is roughly what I do. The one thing I would not economise on even for a station bike is brakes, because those are a safety item rather than a convenience.

The extra money almost never goes into the frame in a way you would feel. It goes into wheels, brakes and drivetrain, and those are the three things that determine whether the bike is annoying in February. Hand built or better factory wheels stay true instead of needing the shop twice a year, brakes that work in the wet stop you rather than making a noise, and a drivetrain a step up shifts under load and lasts longer.

For a station bike specifically I would spend the difference on hub gears and a fully enclosed chain if you can find it used. Salt and rain eat exposed drivetrains and a hub gear does not care.

The threshold being so consistent points at fuelling and duration rather than a structural problem. Under two hours you can get away with nothing on a 16km, but by 18 to 22km you are past two hours and running on empty, and tired calves cramp. Try taking something with carbohydrate and sodium from 45 minutes onward, roughly every 30 to 40 minutes after that, on every long run rather than only the very long ones. That single change fixed the identical pattern for me and for two people in my club.

Coir gets dismissed but a good coir-and-latex layered mattress is well suited to that climate and is what my parents have had for over a decade without a dip. The problem with yours is that it is an unknown builder-grade one that has been compressed unevenly for years, not that coir is bad.

And a clearance trainer at 70 is often a genuinely good shoe from last season, so option two is not automatically worse if you have run in that model before. It is worse if you are guessing.

If you can find a shop that lets you run in them on a treadmill for ten minutes, use that rather than any review. Fit is the entire variable and reviews cannot see your foot.