Many people who face this kind of price tag wonder if they can get away with patching their pipes. But if the sewer line is old there are almost certainly drain repipe other bad spots that will give you problems now or in the near future. A series of patch jobs isn't cheap. In the long run, it just makes more sense to replace the entire line.
There are two, one on each side of the toilet, closet bolts which hold the toilet to the floor. If there are caps over the bolts, remove or pop them open. Using a small crescent wrench unscrew the nuts from the bolts. Alternate side to side when unscrewing this will be much more important when reinstalling the water line repair bolts but alternate anyhow.
One other common design that is considered "washerless" was made famous by Delta and copied by several other makers. It uses little "cup" seals that fit over springs recessed in the back of the valve body. The springs press these "seals" against the cartridge and regulate the flow through little tapered openings in the cartridge. If you have one of these and it's dripping, changing these springs and seals will usually correct it. Here's a tip. Once you have the cartridge out of the way, insert a phillips screwdriver into the seal and pop both the seal and spring out. Stick the new ones on the end of the screwdriver to help guide them into place. This is especially helpful if you don't have long slender fingers.
Working on the gas lines is also a complex job that only a licensed plumber can handle because it is very dangerous. Small leaks can't always be detected by odor alone, but a licensed professional has the proper equipment that's needed to ensure that you don't have any leaks in the line.
The water line repair most likely damaged your electrical outlets and lighting fixtures. Have a qualified electrician come in to evaluate and repair the electrical.
And the way to know what's sufficient? Well, until the work is done an the bill paid you won't know for certain, but this is the best that can be suggested and normally works very well.
Windshield repair techs come and go on a daily basis. The majority struggle to survive. Most fail. It's not that they're not good at what they do, it's not that they don't have the skills, it's not that they're not willing to work hard, it's their lack of MARKETING Training. That's the bottom line: It all comes down to proper marketing strategies.