What are the best options for making an SBS in terms of reliability, durability of parts and availability of replacement parts?  I've been looking around at the local pawn shops for a good candidate at a low price and found an Ithaca 37 Featherweight for $175.  

I'm open to SxS's, O/U's and pumps.

If you're wanting parts availability factory and aftermarket the answer is Remington 870. Mossberg would be #2, I'm thinking about a 14" 590A1 myself.

I may be wrong, but I presume this would be a gun for serious purposes rather than fun? I wouldn't SBS a gun (time + expense) I didn't plan to use as an HD shotgun, but that's me. I prefer a factory barrel so a Remington 870P or Mossberg 590A1 are my recommendations.

You can buy factory 14" Remington barrels at botach and Summit Gun Broker has PD exchange 870Ps for $339. A 1 shot mag extension gets you 5 in the tube. If you want to go the Form 1 route, that is what I would do. You will have the PD shotgun to use while you wait on the stamp.

PD Trades

The 590A1 factory 14" can be found on Gunbroker for around $500 and it has a 5 round tube. It would transfer on a Form 4 so you would not have it until you get the stamp. It also could require a Form 3 transfer between dealers if you can't find one locally. This adds additional ATF wait time.

One seller has several 590A1 that were PD Less Than Lethal guns. They come with a Surefire light forend (don't know model) and were $499 Buy It Now. A new 14" 590A1 is the best value as far as I can see and they are available.

Unless you can DIY or have a very cheap gunsmith, you will end up spending almost as much on that Ithaca as a factory Mossy. If it is an HD gun, I woudn't do it. I prefer the factory SBS and hope mine clears the ATF process as quickly as possible.

It depends on how short you want to go.
A regular Mossberg 88 or 500 can be chopped in front of the lug to make a 14" gun without any other work
A Remington 870 can be chopped to 12" with out any other work
If you want to go shorter the tube has to be shortened and the lug relocated. The mossberg is easier to work on since the tube can be unscrewed, the Remington is soldered on.  I've built a 7" mossberg and a 12" mossberg. As long as you have a welder they are easy to do, a lathe makes it easier but I did my first one with out one.

I've also chopped a sxs Stevens 311 20ga down to 9" barrels. Its a nice looking piece but not really as useful as the little pumps

I would have that Ithaca chopped to 18" already.  But the 870 wins most versatile, best looking,  Miss Congeniality, etc...

I bought one of those Moss 590's.  What do you mean when you said:

"One seller has several 590A1 that were PD Less Than Lethal guns."

Thanks,

They were police guns dedicated to shooting "less than lethal" bean bag rounds, set up with orange furniture (to ID as non-lethal). He swapped out the orange for black butt stocks and Surefire forends.

Thanks.  But I assume they're good to go, full capability shotguns now right, as only the furntunire was different?

They could always have been full capability, they were just utilized to shoot a bean bag round. They probably did not see a lot of use, or any hard use at all, and they should absolutely be good to go. Like anything used, you will have to inspect it, but PD trades are generally thought to be a sound purchase.

Although your FFL can't give it to you before the ATF stamp comes back, you should be able to visit it once he receives it.

My intent was more for fun.   The need for replacement parts would be so I don't spend the tax stamp money and have a broken part ruin my fun.

Random 870 I traded for, upgraded internals, Mesa sidesaddle, 12.5" barrel (shortest with a standard mag tube) Hogue short LoP furniture. I just need to get a XS BigDot Tritium Bead installed

why not do a benelli entry with 14 inch barrel ? benelli m1 entry can be had for 750

Who has them for $750? Everyone I can find costs a lot more than that.