You're doing exactly like my cousin did when deciding whether to buy a new van for his business before or after the first of the year, or to lease, or to keep the old one and do the maintenance. The deciding factor came down to the new one was rated at one mile per gallon better fuel mileage. After calculating for a day and a half, all his time was wasted after he found out the new van was getting one mile per gallon less than his old one. Same brand, same model, same size and same engine, just two years newer.

First of all, no shop is going to want to install parts you bring in. If they do there is going to be no warranty on their work related to them. That is partly why they mark parts up from their wholesale cost. It covers part of their labor if one is defective and they have to do the job over again. If your new water pump howls or leaks, you will be paying a second time to have the work done over so what did you gain? Imagine bringing your own food to a restaurant and asking them to cook it for you, then you aren't happy with the quality. There's a reason we pay more for food at a restaurant than we do to get the same thing at home.

How much is your time worth? How many hours have you put into researching prices, and how many miles have you put on visiting various shops? Part of the parts price markup includes the cost of someone running to get those parts, and they already know where the best deals are in terms of quality, price, and warranty. They want the best quality so they don't have to do the job over.

You're right to have this job done soon before a problem develops. In the '80s Honda recommended replacing the timing belt at 75,000 miles and they commonly broke at 60,000 miles resulting in a very expensive repair bill to replace bent valves. There are a lot of good repair shops all over but this is one job that I would give more consideration to the dealership's repair department. If they mess it up and damage the valves, they will have the special tools needed to make the repairs. There are some shady shops out there, and unlike other professions, they give the entire industry a bad name. Look for a smaller shop that is busy with customer cars, not with dealers' trade-ins. Some of the better shops display their mechanics' ASE certifications, and they require them to wear uniforms with patches showing those credentials. I would be more suspect of a shop where everyone was running around in grubby blue jeans.

There are a lot of variables that go into the prices you were quoted. The lowest one is rarely the best value, and that's what you're trying to determine. You are right too to be concerned with not buying one part now and needing it later. Most people don't think that far ahead. The water pump and belt tensioner are two perfect examples where one shop will include all those things to insure the quality of the repair, and another shop may not include them so they can give you a lower estimate than their competitors to win the job. Often they realize later those parts are needed or they at least SHOULD be replaced, then they surprise you with more things after the engine is apart. If you decline those additional things they find, they can use that as an excuse if a problem does develop.

Replacing the water pump means draining and refilling the engine coolant which is never a bad idea. Some shops will want to do a cooling system flush but I have my doubts about the benefits of that vs the cost. The main reason for replacing the coolant is to get the acids out that naturally form in it and to replenish the additives in it that wear out in about two years. Those additives include water pump lubricant and corrosion inhibitors. Corrosion inhibitors reduce the chance of a leak in the heater core and radiator. If you insist, they will reuse your old coolant but I don't recommend that either unless for some reason the coolant was recently changed. That's being penny-wise and dollar-foolish. Just have them put in a gallon of new antifreeze and a gallon of water.

If the serpentine belt is still original, have that replaced. It has to come off any way so there won't be any additional labor cost, just the cost of the belt. One problem with giving an estimate with this type of repair is there are going to be unknown things that are found once they dig into it. Look at your most expensive estimates to see if they included some of those potential surprises. When giving an estimate from the book, most shops use a "flat rate" guide so they all charge the same number of hours for the same service. If you don't understand how that works, I can describe it for you. Those guides will always list the extra items as an extra amount of time, not as the entire time if just that one part was being replaced. When writing up an estimate, the people don't know which of those things to include. They may have to wait for the mechanic to disassemble enough of the engine to know what is needed, THEN they can give you a more accurate estimate, but the work has already been started, so you're stuck there. That's why some shops include all those potential items, then hope to surprise you later with a bill lower than expected. On the flip side, they run the risk of not getting your business because their competitor gave you a lower estimate. That's where the lower estimate might not be the better value.

Ask what kind of warranty will be on their service. I would also mention that you are aware that there are a lot of aftermarket parts out there that are just as good or better than the original manufacturer parts but at a lower cost, and I would ask them to use their judgement on which ones to install. That shows you trust them and you're willing to pay for the peace of mind. They aren't going to go with the cheapest if it means there's more of a chance they will have to do the work over if a part is defective. It also will make them more willing to search for the best deal for you.