There's no such thing as purchasing at first sight. Purchasing takes time, and purchasing takes work.
Fate doesn't operate in videogame terms, it operates in narrative terms. Getting a set of Top-of-the-Line Exploration Gear is more complicated than walking up to the single merchant in the hub area, scrolling to the end of their inventory, and clicking on the thing with the largest price tag beside it. Your Fate PCs are capable people living dramatic lives, and getting gear that's a significant enough upgrade to their capabilities to be worth an invoke is going to be a similarly dramatic affair. Phone your suppliers, take fine measurements, weigh how trustworthy the offers are, make sure nothing you've bought is going to work at cross purposes.
Make a scene out of it, in other words. Every roll you make in Fate needs to be set in a scene, and that's the sort of scene you'd need to make to create that kind of long-term advantage with Resources. It's a sort of "summary scene" abridging longer actions, that happens to set up the actual plot - you don't let someone just freely stage it and rack up twelve free invokes on gear, any more than you let someone grind down their weapon and stack twelve invokes on a Finely Sharpened Sword, or meditate on their center and stack twelve invokes on One with
Crucially, it's not exactly the sort of scene you can productively set while Perducci's getting away with the serum, after him!
Resources in Major Cs
Let's suppose the PCs are in some kind of Contest. It's a bit unusual that Resources would be used for a primary skill in a Contest, but it can certainly play in as a secondary skill where Moneybags elbows to the curb, flashes a wad of bills, and Taxi! to get an invoke or two on the Notice roll they're making to follow Perducci's trail. Because flubbing a secondary skill sinks the whole round, even throwing a high number still carries that risk.
Or perhaps they're in a Challenge, where they've cornered Perducci at the docks, but then he takes a flying leap into a speedboat bound for Isla Remota. Everybody in the party has to step up to the Challenge, and everybody's probably going to square up to the Challenge with one of their best skills. Even if Moneybags taps his Resources and charters a helicopter out to Isla Remota, that doesn't eliminate the need for everyone to participate in the challenge, so come up with something for them to do, like Shooting down the swarm of laser hawks that come screaming up from Isla Remota.
Conflicts you've already taken the measure of, right? Can't really drop Resources on somebody's stress track, which is where the idea of purchasing advantage for the players entered the picture in the first place.
Resources and Stunts
The general pattern of "once per session, achieve a 1-shift success in a wide area" is useful to apply here, and so: sure, that's a decent idea for a stunt. If it was up to me I'd break it down just a little differently, as:
Where Does He Get Those Wonderful Toys? Once per session, when you have time and access to your enterprises, you can create an advantage on yourself and each of your allies called Top-of-the-Line X Gear with one free invoke. The X is one of: combat, exploration, investigation, fashion, and everyone can pick it as they'd like. You can take this stunt multiple times, but people have to pick a different X each time. Progress is cruel, and the advantage will go away at the end of the session if it isn't lost before then.