Read my last post about the Tstat. I wouldn't give more than $30 for parts and labor for the mechanic to switch it out with the timing belt.
Your air filter is not held on by four spring clips, but by four bolts (machine screws).
Any mechanic worth his salt should have broken fastner extractors and know how to use them profeceintly. He should be able to remove and replace the parts within 10 minutes, including slapping a new air filter in.
Yeah, you're on a small budget. Take this lesson in life at this age. Start an emergency fund for stuff like this and never use credit cards.
Your oil leak shouldn't be that bad. I've seen 1/2 ton trucks very overloaded, with more than 250k miles on them, that burn and leak a quart of oil per day run for over a year like that.
Keep the oil full. Buy a case of the cheapest stuff you can find and stick it in the trunk. Check it every morning or afternoon.
See if the shop will work out a 60 payment plan with you. See if you can pay 1/2 and let you pay 1/4 each month.