Best Coilover IFS for '71 duster?
This is what I would do (and have basically done on my car...)
1.06 (or bigger) Firm Feel torsion bars - $355
Tubular UCAs (PST, FFI, etc) $350
Adjustable strut rods (PST SR 14385) $279
Solid tie rod sleeves (PST SAS 440S) $49
LCA boxing plate (PST LCAPLTMOP621) $14.50
Greasable LCA pins (FFI w bushings) $135
Hellwig tubular front sway bar 1 1/8 #55906 $231
Hotchkis Fox Shocks (non-adjustable) - $280
That puts you at about $1,700 for what I have listed. You'll still need to add a set of lower ball joints, tie rod ends, and LCA bushings. Everything else gets replaced with the new parts. That's just for the front since you're looking to go IRS in the back.
For steering, the biggest power upgrade out there is the Borgeson/Delphi steering box. Bergman Auto Craft sells them for $695, but then there's some additional costs for the steering coupler and lines. I'd contact Peter for a full run down on that. He's GmachineDartGT on here.
http://bergmanautocraft.com/
Or you could just add a steering quickner to the stock power system with a rebuilt box. Howe sells the 1.5:1 steering quickners for $90. Rebuilt power steering boxes are around $360 from Firm Feel. You'd have to do some custom work to install the quickner, but it's not a huge deal. That's the set up that the Hotchkis Challenger runs, and that would get you a 12:1 steering ratio.
Brakes vary quite a bit depending on what you're looking for, but really the brakes are going to cost about the same whether you stick with the stock set up or opt for the coilover conversion. Personally, I'm a big fan of Dr. Diff (Cass Eslick). He has some great brake kits on his website. I run his 13" front "cobra style" disks, which will set you back $850, assuming you already have the later 73+ disk spindles.
Assuming you did everything, including the Borgeson steering box and the 13" rotors, you'd probably be looking at $3600. But that's allowing $1k for the Borgeson/Delphi box and adaptors. You could do a stock rebuilt box and quickner for about $3,100 all in, allowing some money for the steering column mods.
The RMS alterkation starts at $4,250, and that still doesn't include brakes. If you add the somewhat compareable 4 piston 12.19" wilwoods, you're looking at $5k. Not sure how HemiDenny's kits compare to that, he would be the one to ask. I thought his were a little less, but again, he'd be the one to talk to.