Dude
You probably already know this but....
Lesson 1. You very rarely get something for nothing, every now and then you may get a 500 quid car that runs for a couple of years and needs nothing done. This won't happen very often. '13 year old fiesta needs some work to pass MOT'; not exactly a newspaper headline is it?
FWIW I'd have taken the micra every time. Forget that crash rating, unless your planning on smashing it up every week it isn't going to matter is it. Do you honestly think you'll walk away from your first kerbing at 70mph thinking 'I 'm glad I got the festa, it felt really solid'? It won't matter much on your insurance either. Also, I haven't seen any 10 year old micra's with rusty sills but I will look out now you mention it. BTW they're built in Sunderland, not Japan where they don't salt the roads so they shouldn't be too bad for rust. Okay; it isn't the best looking car, but lets face it, that fiesta isn't a styled by Pininfarina either.
Lesson 2. Next time make a big long checklist of things, (including all MOT check items) and work your way round the prospective car from outside in, back to front or vice versa. Remember to check things like Oil + other levels before you start it. Make sure you get under it, (ideally on a ramp / set of ramps), poke around, get a torch in under the wheel arches and underneath it at least. You can get a good feeling for wear from doing this, you'll spot any leaks or the need for new bushes pretty quickly too. Take your time, if nothing else you will look like you know what you are doing and may be in a stronger position when it comes to saving yourself a tenner on the price because the spare tyre is bald etc.
Test drive it, ALWAYS. Give it as much of a going over as you can with the owner sitting next to you / behind you, don't let them talk, sit them down and listen. Check everything you can on drivetrain, gears up / down, reverse. Braking into tight bends left / right. Find a car park, nearby and do some sharp figures of 8 to test full lock, listen for noise. Rev through the range in each gear, don't have to red line it, but get close. Change from 5 to 2 and other changes. See where the biting point is and how big it is too. Check the brakes, no starsky and hutch stuff but make sure you do it properly, feel for resistance and 'bumpiness'. Make sure all pedals return when you release the pressure. Don't forget to do heater / air con, radio, internal lights, mirrors while on the go either. When you park it back up check for excessive moisture at the exhaust and flip the bonnet. Any major issues / leaks should be obvious now.
If you don't know how to check or what to look for then make friends with a mechanic. Or do some reading.
Lesson 3. Once you've bought it:
1. Buy a drip tray / tank and learn how to change the oil and filter. It is a piece of piss on most vehicles. Do this every 5000 miles.
2. Use some of that fuel cleaner every now and then too.
3. Buy a plug spanner and change them when you do the oil.
4. Check all the MOT items before you send it. Wipers, Lights, Seatbelts can mostly be sorted yourself without paying somebody else 30 - whatever quid an hour for it.
5. Accept that certain bits are just going to wear out. Distributor caps, pumps, other high friction parts, exhausts etc will all go at some point. It is just a matter of time.
Okay, this won't catch everything but will keep the catastrophic engine failure wolf from the door.
Lesson 4. I'd ditch it, sounds like a dog and it won't be worth what you've spent on it when the work is done. Sell it to a scrappy for 100 quid and save for a couple of weeks to buy something else with the 300+you've saved on repairs.
Good luck, and remember a car isn't just for Christmas.