Around here, once past the meter, you can do any work without a plumbers license and the only back flow preventers is at the hose bib and are really just a vacuum breaker. If anything is at the meter it is on the county water supply side. I do have a huge one for my sprinkler system that is required but it is tied into the main line before the line to the house.
If the line is 15-20 years of age, then I would replace it when you have some good weather and a weekend free. Likely that it will give up the ghost when in the dead of winter or during a storm or sometime when you are gone is pretty high according to Murphy's Law. IF the water company said it needed replacing, I would go with their advice and replace it. They have no dog in the hunt to give you false info on this as it is your responsibility to keep it up and any water loss due to a leak is yours to pay for. Personally I wouldn't use PEX if you have any rocks in you soil as it is pretty soft. I would go with schedule 80 PVC which is good for 350 PSI and gives you an extra margin of safety, its cheap @ around 30 cents per foot and all the fitting you need are readily available at your hardware store. Use the blue glue and you don't need a primer and it is not affected by water like the clear stuff so if you have a little drip from the line, it will still work and dries almost instantly so no need to wait hours prior to energizing and checking for leaks. I used it for all of my sprinkler and adding in some additional faucets for my brother in law involved about 300 feet of new pipe. We had no leaks and tested it within 10 minutes of completing the last joint and this system runs at over 100 PSI from the water company. We have pressure regulators before it enters any building but the outside hose bibs have full system pressure.
