I think you and I have the same idea. I'm looking to get low w/ the greatest amt of comfort. Just swapping springs you will lose ride comfort, and may not gain any performance. If you go lower than 2" you definitely have to add Bumpsteer kit and it's STRONGLY ADVISED to add quality caster/camber plates. And even then, your suspension geometry is going to be out of whack.
If you intend to go 2" or lower (IMO it's not worth dropping if you're not going @least this low! I hate tire/fender gap...) you have precious few options: air bags, coilovers or custom components.
For Air bags, check out:
http://www.ridetech.com/catalog/Ford_Mustang_1979_1993-990-1.html
I've talked to these guys and they're pretty cool. Apparently you don't have to do the whole compressor/lines thing. You can just have the air springs and get a Schrader valve (think: bicycle pump...) set up and be low and have Lincoln comfort for like $900. Of course, you're still gonna need the bumpsteer kit and the caster/camber kit is still a really good idea.
For coilovers, we have two good options and a bunch of cheapie, knock-off options. FOR ME, suspension components is NOT a good place to cut corners. Everyone on here and any other mustang forum I've checked stands by the following options: Griggs Racing, and Maximum Motorsports. The main cheapie option that you'll see are the UPR coil-overs.
When looking @ coilovers, you gain performance and up to a 2.5" drop (although you may not get as much performance w/ them all the way down b/c your suspension geometry is jacked). You're still gonna need the c/c plates and bumpsteer kit to mantain control. Most report that they are @least as comfortable as factory.
What I'm seriously considering is going custom.
http://www.racecraft.com/proddetail.php?prod=300103
Bad news:These are $500, and they only lower the front.
Good news: you don't have to get the c/c plates & bumpsteer kit, you keep your suspension geometry as designed by the engineers.
I go back and forth a/b combining these w/ the air bags or just cutting the factory springs (less than 1" of cutting seems to be accepted practice that doesn't affect ride quality or performance). Team either of these options w/ upgraded struts for good front end comfort and a nice drop.
In the rear, I'm looking @ 2 custom options -the one I'm leaning toward is swapping my Solid Rear Axle (SRA) for an Independent Rear (IRS) from a 99-04 cobra. Apparently, the guys that drag race these cars will often swap straight up (or for small $$) their IRS for our SRAs. MUCH better ride quality and some have said they add more clearance. Team this w/ a rear coilover kit/air bag set and you can get down pretty nice.
The other option that I'm looking @ for the rear is a j-shaped Lower Control Arm (LCA).
http://www.jegs.com/i/Granatelli/467/GMCA7998WJ/10002/-1
These are a/b $200-$300 and lower mounting point for the rear springs by an inch, and then I'm looking at a mildly cut 4cyl spring or a set of air bags.
Of course, to get this low you've gotta pay.
$500 for spindles, $200 for LCAs and then the shocks/struts and either the 4cyl springs or rear air bags ($500 for the CoolRide rear kit w/ shocks). All this I'm budgeting a/b $1200-$1500 for my drop on my daily driver.