Coolant spilling nightmare

The car in question is a 1997 saturn sc-1 with approx 140, xxx miles on it.

last week I was driving this car to deliver pizzas. The car had been running for an hour or 2 ( I leave it idling between deliveries) when I started driving it and it began steaming from under the hood, at the coolant reservoir. I got out of the car (in a parking lot) and noticed that all of the coolant in my car had leaked out. I turned the car off and returned to it the next day.

the car is my girlfriends, and I asked her if anything had been wrong with it lately. She said the temp gauge in the dash had stopped working a week or so prior, so I replaced the thermostat. The car drove fine for 2 days.

then one day, after the car had been running for an hour or 2 again, it spit coolant out all over the parking lot at my work and started steaming again. We popped the hood and once again the steam and coolant appeared to be coming from the cap on the coolant reservoir. I replaced the cap, thinking it had lost pressure, and drove it again the next day. Once again after about an hour and a half the coolant and temp indicator lights started flashing in the dash, and about 15 minutes after that it dumped all my coolant again. This time we found a leak under the car, in addition to the reservoir cap. We do not think it was coming from the radiator, but were not sure.

according to the temp gauge, the car is not running too hot (right at about 1/4 of the way up after running for an hour). The car drives fine with no unusual noises or anything.

just to get the car to somewhere we could park it, I waited for the engine to go cold and re-filled the resevoir with water and sent my gilfriend home in it. Now, this may be unelated, but when she started it up, the rpm gauge dipped below 1, then shot up to 2, sat there for about 5 seconds, and then would drop back to 1 where it belonged. I dont know a lot about cars, so I dont know what the hell that could mean. Anyway, after about a 15 minute drive the coolant and temp indicator lights started flashing again, and by the time she got home the car had dumped more coolant into her parking lot.

here's my questions:

is the car safe to drive 15 minutes to a repair shop?

what different things could be wrong with it?

how much time / money for parts would these problems run me to fix myself?

how much should I pay at a repair shop?

thanks a lot in advance.