Sorry for the late response-- I don't come by RC too often-- it tends to suck up too much of my free time.
Regarding the ASOV, it depends-- if you buy one for drinking water, these are usually built into it (you won't see it) -- many other ones for sale have them as an "option" before the unit-- I bought my SpectraPure unit used for $75-- it did not have one so I bought one, some plumbing fittings, and a pressure tank from PurelyH2O.com to convert mine over for both.
You can see my ASOV in the first picture-- it is the white thing about 2 feet below and slightly to the left of the unit.
Since you are using Well water, IMO you should spend a little bit extra for a quality unit-- this will eventually pay for itself in reduced DI resin use-- however, the time it takes to recoup your investment will depend on how high your TDS is and your usage.
If you look around the equipment/lighting section I'm sure you will see plenty of recent RO/DI threads-- any recommendation by AZdessertrat is what I would buy.... I haven't been around RC alot lately so don't know if he is recommending anything different then what he used to....
He used to recommend-- the "optima" from Purelyh20.com
and
the "typhoon III" from airwaterice.com
If you want a dual drinking/reef unit -- after deciding on which you want-- I'd consider calling one of them up-- and telling them exactly what you want to do with it and what you have (well water)-- for this reason alone I would maybe go with the Typhoon III as it has an extra prefilter stage-- that being said, I had no problems running mine on well water with just 2 filter stages before the unit -- I believe one or both companies offer a discount to Reefcentral members-- once again-- I'm out of the loop-- I'd start a new thread if you want some "new" opinions-- Spuds is out of the loop.
I'd figure on spending $150-200 on a new/quality RO/DI unit and another $50 or so for a drinking water add on kit (pressure tank, faucet, fittings, ASOV)
One last thing, you will see some units have 100GPD or 75GPD units-- get one with a 75 GPD Dow Filmtec membrane-- DO NOT GET THE 100GPD dow filmtec -- the 75 GPD RO membranes reduce your TDS by 98%-- the 100GPD ones only reduce it by 90%-- your DI stage will remove any remaning TDS from your water--- so this means your resin will last 5X longer with the 75GPD one then with the 100 GPD one-- the 75 GPD membrane is plenty... also the DOW membranes will work better with lower water pressures -- which you will encounter with a well pump... with lower pressure you will experience less production-- so if you are running only 50 PSI or so, expect your production to be about 40-50 GPD-- I only got my unit to produce its capacity after I moved and switched to city water running at 75 PSI.
HTH..
Bill
PS-- I know I mentioned I am using city water-- yet you can see a well pressure tank in one of my pics-- I believe my house used to run on just a well-- it now has city water. The well supplies my outdoor faucets for watering and stuff-- it has alot of sulpher in it and stinks alot-- I won't even let my kids use it for their summer pools/sprinklers-- I run a hose from my laundry room outside for that stuff.