Big box stores have spoiled people when it comes to plumbing shopping because they get the manufacturer to combine everything needed including the internal valves in one box. In most plumbing stores and special orders from big boxes, the guts of the roman tub faucet are sold separately from the pretty pretty parts, and the brands don't have interchangeability. In most lavatory faucets, the valves and trim are not sold separately. They are sold as a unitized structure, with maybe only the handles themselves being an optional choice. I'm not at work right now to look at the Moen catalog and see which category they fall under, but I suspect that you will need to purchase all new everything for your sinks, shower, and tub.

1) can you see the plumbing anywhere else? They probably didn't use 3/4" pipes if they didn't elsewhere in the apt. 2) they probably used galvanized pipes, not copper, so some replacement will be necessary anyway. 3) If you really want to know, break out a bit of wall--you're going to have to do it anyway to replace the valve. 4) You probably will want the 1/2", unless you're doing a shower system and need the extra flow. 1/2" is enough for quite a bit more than the flow-restrictor's 2.5gpm

quote" I'm thinking a three way valve would be easier to install, more reliable, and cheaper. "quote easier to install- no more reliable- yes cheaper- no 2 3-way valves will give you the same options as a slide valve, filter or backwash only. Vacuum to waste would take a third 3 way. That is the only real benefit of a multiport valve over a slide or two valve setup. quote" Do I really need to backwash? "quote Only if you want to extend the time between filter cleans, and have less to clean off of your grids when you do breakdown the filter. I always backwash the grids as effectively as possible before taking my filters apart. See ya, Kelly

Soldering copper pipe is a good homeowner skill to learn. And, it's easy to do. When you buy the torch, pick up a short length of pipe, some fittings to practice with, and a soldering kit like this: https://www.menards.com/main/tools/torches-soldering-tools/oatey-reg-safe-flo-silver-plumbing-solder-kit/50683/p-1444449926798-c-8533.htm?tid=-8460164875200987733&ipos=2 Clean the inside and outside of all the pipe surfaces where they would overlap with the sanding cloth, surfaces should be bright and shiny copper. Apply a small amount of tinning flux to all surfaces, assemble things and its ready to solder. Turn on the torch and set it on a level surface in front of you. With channel lock pliers, hold the assembly with one hand and bring the assembly into the heat of the flame. Not into the base of the flame, stay more towards the flame tip. If you overheat the joint, the surfaces oxidize and you're done, the joint will never solder properly. Bring the joint temp up slowly, the tinning flux will start to bubble, and with the roll of solder unwound/extended and in your other hand, test the joint with the solder on the opposite side of the heat. As the joint comes up to soldering temp, the solder will melt and be drawn into the joint by capillary action. The joint is done! You can wipe the joint with a cotton rag to clean it up if desired. As the joint cools the solder solidifies. You can touch/drape a wet cotton cloth around a joint to speed up its cooling, or if I'm preassembling several fittings at a bench, I like to drop the assembly into a bucket of water so I can handle things quicker.