I've spent a lot of years owning and having fun with M-'s and F-'s and a J-' thrown into the mix.
Rust issues were pretty much figured out by mid '77. From there on up a rust free F- is like coming across a rust free A- or E-. The biggest thing to make sure of is the cowl drains being open and them to do their job.
There's a lack of parts availability, it's true, so you have to make a network for parts and beat the bushes. What a lot of us had to do once upon a time before all the restoration parts came on line for A-'s. Hell, some of the fun was hunting down parts and gaining those connections. For that, though, there are guys on FMJ's who have the parts you need for a price that's affordable.
The only drawback to the platform is a ton of rubber in the suspension. All the iso mounts, etc. It's all easy to eliminate and places like Firm Feel have the parts you need to help bring back road feel. There's also stiffer urethane parts out there that can be used to replace the rubber on the cheap without breaking the bank on "specialized" parts.
From there, the sky's the limits. Sure, they were produced during the smog era, but they don't have to be put back together with smog era parts in most states. A 360 you'd build for an A-body is the same 360 you'd build for any other car.
No hunting down disk brake conversions; the F-, M-, J- platform came with them. Big bolt pattern already there. Power steering, check, with quick ratio AHB boxes available. 8.25 rears with disk brake upgrades from a Jeep easily enough.
Pre-'71 B-body 8.75's are pretty much a bolt in and easier to find than 8.75 A-body rears.
I've got a buddy who just sold his factory '79 E58 powered Road Runner for almost $20k. That's about the same price you'd be paying for a fully restored A-body comparably equipped.
Another buddy is building a 413 powered '79 Road Runner with a Corvette front suspension, IRS from a Cobra, and twin turbos.