The Conshelfs are bulletproof, almost. The first stage of this model may still have the old style high pressure poppet but it probably has one of the newer types. This was a transition period. The old poppets will leak if the tiniest bit of grit or a sliver of chrome gets under the seat. The really old Conshelfs had poppets with neoprene seats which did not hold up with high pressure, they would delaminate. This just means that the regulator, like all older models should be inspected. The tech might need to install a new, updated poppet and some other bits and pieces. However, before deciding, you can take a look at the copper filter. If it is shiny clean, the regulator 1st stg may be good to go. The extra ports on the reg use industry accepted thread sizes for modern HP gauges and LP attachments, no problem there.
I don't particularly like metal second stages. They are a trifle heavy. However, if the mouthpiece, diaphragm and exhaust valve of the regulator are not stiff or checked and the unit breaths OK on the surface, it should be useable. This is easy to examine by removing a clamp. The Conshelf 21 "select" had a plastic 2nd stg. I'm not sure about the other versions. Same remarks apply.
The Conshelf 21 is a competent regulator in that it will deliver adequate air for recreational diving purposes. I've used them and they breath fine. Don't misinterpret the technical remarks above, a Conshelf in good condition will give years of service.