Which isn't all that big a deal, but as is often the case with plumbing repairs, the tricky bit is the prep.
One drain was leaking, so I had to fix that first - definitely the easy part of the process.
Next step was to install the missing undersink shutoff valves, so I didn't have to go downstairs and shut off water to the entire house every time I wanted to do anything. Since one's hands are supposed to be relatively clean when installing a filtration system, it seemed a good idea to be able to wash them somewhere in the house.
For some reason, the baseplate on the old faucet was all but welded in place (partially by time and hard water deposits, but the overzealous previous owner definitely helped). I had to cut a bolt in 4 places with the dremel before it could be pried off.
Most sinks have 2, 3 or 4 holes. Mine had 4.5 - apparently the previous owner had drilled a special tiny hole for the old filtration spigot. After some pondering, I offset my new faucet on the double sink, in the leftmost hole, with the handle attached on the left (and the intake hoses switched, so cold is still foward and hot is still back). Center hole got a hole cover, next hole is where I put the spigot - also left-handed handle, so as to not be crowded up against the dishwasher thingy, Ex spigot-hole got another hole cover. After using this setup for a week, i"m pretty happy with it. New faucet is one of those tall sprayer-in-faucet models - it easily reaches both sinks, and accommodates every stockpot.
The instructions for the RO system looked reasonable, but the steps were in a very unfortunate order. Step 1 should not be "drill a hole in the drain pipe", because if you do that and later find some reason that you must stop...you can't turn the water on until you repalce the pipe. Based on evidence, step 1 should be "identify what pieces you need from the hardware store to install this other piece in your cold water line". It should have been very standard, since it was to go in between the new shutoff valve I'd just installed and the new faucet line. It wasn't.
Step 9 references "step 6"...which is actually step 7. One of the first steps is to install the tank so as to block all subsequent steps. And so forth.
Thankfully, the undersink area had a lot of room. Still awkward of course (bruises and a twinge-y back when I was done), but I can't complain. Except that there weren't any solid bits of wood within reach, for installing the set of filters. For solidity and access, I installed them on the cabinet door. Which is great...except that Z tells me that cutting the one piece of wood I did cut is against code. Well, I'll shore it up just in case it does serve some purpose.
RO system is to be sanitized with 3ml of bleach at end. It assumes regular-strength bleach, but most store-bought bleach these days is "concentrated", so either you do the math or you do a second round of flushing to get rid of the bleachy taste.
Final indignity: while inspecting the RO setup, I snapped off the tiny protuding bit on the RO filter. $60 down the drain, and yet again still another trip to the hardware store.