Records not stored vertically for decades

I've had records that were stored flat for months to years and never had an issues with them.  I think as long as they weren't submitted to excessive heat or humidity they'll be fine.

So since you have about 200 albums, I would run them all thru a US RCM. That will get rid of any mould/dirt/dust, etc. The only ones I would worry about for warping are the top 5-10, since they would not have much weight on them. Vinyl is heavy, so anything under those would be fine from any warps. 

My 2 cents... Good luck and enjoy when you get everything back up and running. Lots of fun. Just finished listening to Analogue Productions Gaucho - 45 rpm - re-mastered by Bernie. Another home run if you are a SD fan.

The issue with storing records flat has already been brought up. Primarily, the increased potential for warpage and sleeve damage. This will depend to some extent on whether the record in question is a box set or not, generally a box set is more durable as to the contents. Easiest thing to do is just set the record on the platter and do a little spin, this should tell one if warping is present.

Just finished listening to Analogue Productions Gaucho - 45 rpm - re-mastered by Bernie. Another home run if you are a SD fan.

@dmk_calgary Funny you mention Steely Dan — my favorite band.  One of the things I was hoping is that my old vinyl sounds better than the digital versions that I find hard to listen to — especially the older stuff.  I have MoFi Orig Master Recordings of Aja and The Nightfly on vinyl that I’m especially looking forward to but heard the latter’s not all that great (never critically listened to it on a good system).  Any recommendations for some really good SD vinyl?

@soix I am confident that your records will be quite well.  The pressure from horizontal storage no doubt pushed contaminates into the grooves.  But that is the only real issues you need to be concerned with.  No matter how you cared for them many years ago, you definitely need to use a more sophisticated cleaning system for their new use.  Without having to spend very much, you can get a respectable wet/vacuum record cleaning system, such as The Record Doctor or a Nitty Gritty, and also a stack of plastic anti-static sleeves.  Treat your records right and they will deliver good results, better than in the past.  Acoustic Sounds has the record sleeves you need, and at about 25 to 30 cents per record it's the very least investment there is into the world of vinyl.  Acoustic Sounds also has all of the Steely Dan albums you desire.  From relatively inexpensive reissues to the state-of-the-art UHQR reissues.  Analog is a lot of fun.  Enjoy the newly inspired journey, once again.