Disclaimer: I'm not anywhere near a building professional, and am barely skilled at building related stuff. But I have done a ton of looking into these things over the past several years as we've remodeled on of our houses and have been working on using our barn for shop activities.
Insulation and sheathing aren't lightweight. Before doing anything I'd ask your building mfg what kind of loads the uprights can take. You may need additional framing (wood or metal) in order to adequately hold up additional mass. The amount of weight that the current framing can hold will determine your available options. Doesn't matter if you can get an r45 wall for $600 if the weight will tear your walls down.
Next, you'll want to know what kind of weight your foundation footer was designed for, because additional framing may help hold more insulation, but if the footer can't take it, then it's moot.
With those constraints sized, you can look at what your feasible options are. How much insulation you need will depend on your goals and climate. R6.5 won't do you much good if you gotta deal with temps under 0, and still may not prevent condensation. Designing an insulated envelope can be tough because you need to constantly keep the interior temperature above the ambient dew point. The more humid your location, the higher the temp needs to remain.
A radiant backing between your metal surface and the backside of your insulation (foil backing) is a good idea. Metal conducts heat quickly and radiates heat much better than wood building materials. You want to ensure that the foil ix permeable though, or that you keep a gap between the back of the insulation and the metal wall so that air can circulate. Even spray foam can absorb water where there are seams and "laps". A good installer can prevent them, but any joints where new spray overlaps cured or partially cured foam can trap water vapor, which becomes standing water.
Since your walls are corrugated, airflow behind the insulation should be easy. The job of your exterior sheathing is basically to keep rain and snow off your insulation, and thats about it. Which means your insulation and any interior sheathing should be sealed to the floor and create one sealed interior envelope. Which also means you want to insulate over the metal beams, not necessarily between them. You can add additional insulation in those cavities, but it will only be marginally effective since any exposed metal will be a point of condensation and will also rob heat from your interior space. If your interior isn't a sealed envelope, warm air will carry water vapor into any place there's an air leak and you'll eventually be dealing with water damage. Even if your materials are waterproof, you may still get mold or bugs, which are no less troublesome. Ants have been known to burrow into foam and even rockwool to make nests. Birds eat ants and like wool-like materials for nests... You can see where this goes.
You can use almost any heat source which vents combustion outside. Direct fired heat sources don't do this, but there's plenty of cheap furnaces or wood heat which will fit the bill. Even an oil burner will do. Just needs to exhaust outside to carry away the moisture. Radiant floor heat is probably one of the best you could install for a shop, but it's typically tough to do after the slab is already poured. Wood fired boilers, heat pump water heaters or any other means of creating heat can be used with it for max efficiency or convenience. You can use electric floor heat, buts it's far less efficient compared to a heat pump driven radiant system.
Polyiso sheets tend to be cheap and if you use two layers (2" ea), staggered. It's possible to get a very tight envelope that should exceed r15 in many cases, which is close to residential requirements in some places. Even rockwool will need sheathing, and probably costs more per square foot than poly iso, and will also require more framing to hold it to the walls since it's made for typical wall cavities (based on what I've seen). Polyiso is also commonly available with a radiant barrier on one side (foil faced) which is still an excellent thing to have in a metal building.
Just my $.02
