I've had my 121M1 for quite a few years now and I really like it. I've never seen a 121 with a Super 90 stock on it though. Around here a couple of years ago a large PD got rid of several of these. They upgraded to the newest widget or whatever.
The 121 came in several variations, mine is the "tactical" version (extended mag tube, etc). When I've seen them at shows lately, the asking price is around $375-$500. In the M1 version I've seen two different stocks; one with a sling attachment through the buttstock and a full plain buttstock.
I've never had a lick of a problem with mine. Have loaded mixed loads (#8, 00 Buck, slug etc) with nary a problem. Parts are out there still but you have to look around a bit to find them.
Another plus with the wood stock is there's always the buttstroke if you ever to "enlighten" someone or something without shooting.
My only problem, which isn't really a problem , is the safety. This SG has a crossbolt safety and I'm left-handed so it's not quite as manageable for me as for all the wrong-handed folks