The missing clip between the air box and the inlet isn't toooo serious. You could have had some grit and dirt getting in there but if your injectors aren't blocked up and you re-fitted the clip then it'll be okay.
The black box is the MAF by the sound of it. It tells the ECU how much air is getting sucked in so it can control the fuel mixture. Again, if it's no longer flapping around then no harm done... though it does have a little sensor on a thin and brittle wire which hangs out in the breeze.. It sits behind the throttle flap but it can be broken if you shove a rag in there too roughly.. so be careful.
The erratic tickover is probably the lambda sensor. You can get new ones on eBay for £25 (for a splice-in job) or £50 for one with a plug pre-attached. Paying £25 for a plug sounds a bit dear but the wiring on these is one-piece so you don't have to join the sensor wires to the existing wiring. It's more secure and the sensor lasts longer, I've found. Buy the best lambda you can afford (probably NGK or Bosch). New lambo' will make your mpg better too, assuming the thermostat is also good.
I'd be worried a little though because with these sorts of issues the beast obviously didn't get much TLC. First thing I would do is get it properly serviced. Give it some new 10W40 semi synthetic oil .. my tip is to use the Halfords stuff for diesels (gold can) as it contains some detergents which will clean up any sludge more effectively than regular "for petrol" oil.. and your fella may have missed some changes or had the same oil in it for years, because it wasn't doing the miles.
Use a new oil and air filter and clean out the breather pipe and the oil neck (tends to get carbon build-up).
) you must change the cam-belt and tensioner unless you definitely know that these were both changed within 3 years/36,000 miles. The Boxer belt is not toooo fragile but again, I'd suspect you could even have the original stuff at that mileage (i.e. 14 years old kit).
Tensioner is just as important as the belt... so don't cheapskate it. You got this far! Some might suggest it's also a good time to change the water pump but at your mileage you should be okay. Water pump is driven by the belt so it's sometimes good practice to change it before it wears out.. saves taking the belt off again and all that labour, that's all.
Change the plugs.. yours may be okay.. but it's a *****-fight of a job so may as well do it all at once.. only take them out again in 2 years time, when you fit new plugs!
C/L problem.. let me know. Mine's a bit lazy to lock sometimes too.. though my lock feels stiff so I'm guessing the motor just can't trip the lock manually so it unlocks after locking. Maybe an oil-up of the mechanism can help.. but as I say.. I didn't investigate mine yet.
You'll probably have doubled the value of the car by now... but it'll be pretty indestructible after this lot. Just don't skimp or it'll likely be a short holiday romance rather than the love of your life..