Scubapro still fully supports the Mk 10 and annual service kits are stocked by virtually every SP dealer as the reg was made for over a decade and is still extremely common. There is no need to pull parts from many other kits.
Scubapro also tends to standardize parts within second stage classes and tends to make any improvements retrofittable into older designs. This means that the G200B uses the same poppet assembly and annual service kit as the Balanced Adjustable, G250, G250HP, G500, S550, and S600. and that older Balanced Adjustables, G250's and G200B's are in effect "upgraded" with the new poppet at annual service.
The new S-wing poppet required a new balanced chamber in many of these second stages and Scubapro included this in the annual service kit up until very recently. If the reg was properly serviced in the last few years, it should have the new balance chamber. If not, one will need to be installed and may need to be ordered if the delaer does not stock them. But the cost is about $1.00 and the part is still available and is a current and standard S550 part.
Scuabpro uses the same service kit for the Air 1, D300, D350 and D400 and still fully supports all of them except the Air 1 either through direct parts replacement or through the ability to upgrade D300's and D350's to D400 status.
The D400 is still fully supported, the problem is finding a tech who understands them and is both willing and able to get peak performance from one as they are different to adjust than a normal second stage. When properly tuned the D400 is, in my opinion, the best second stage ever made in that it is unmatched with it's combination of raw performance, low inhalation effort and natural feel.
If you are looking at buying a used MK 10D400/G200B be advised that a reg with a current service and operational history is preferrable to one that has been used and abused. There are some parts on them that can be expensive to replace. In the case of the D400, the older ones used a one piece aspirator body and a metal orifce that offerred superb performance. But if the orifice is damaged, it's about a $50 part to replace it and you will get the newer 2 piece part with the plastic orifice that offers still excellent but less superb performance. With the Mk 10, the piston sits inside the first stage body which forms the ambient chamber so any corrosion in the ambient chamber that effects the area where the piston ring seals requires replacement of the first stage body which is again not cheap. With the older Mk 5 and newer Mk 15,20 and 25, Scubapro used a shorter main body and separate swivel cap to make it easier and cheaper to deal with any corrosion that may occur.
Also, if you are bidding on a D400, be aware that their exeptional perfromance produces some rabid fans and they often go for a lot more than they are objectively worth, especially if you are bidding against a recent convert who is trying to stock up on them. Unless it were new or minty looking and very little used, I probably would not pay more than $125-$150 for a Mk 10/D400/G200B and then only if the seller could confirm a fresh annual service.
