Tom,
I'll give you my perspective as a dad who put his teenager son in a Ralt RT-41 Atlantic (the best FA of its day) straight out of 2 years of every weekend shifter kart racing.
1. Any youngster who has the physical and emotional skills to
consistently run and finish near the front of a *real* shifter kart field, without crashing into other folks and causing "sportsmanship" issues, can successfully transition directly to an FA. By *real* I mean full race fields, not a few cars at a karting experience. At this point, only you know if your situation meets that criteria, though from the sounds of it, it could.
To a competitive shifter karter, the first experience in an FA was perfectly captured by my son's comments after his first on-track session. When I asked how it went, he replied "Dad, it was like a big, heavy, slow kart...with better brakes." That's right, to a good karter a 1300 pound Atlantic will feel like a fat pig at first. Yes, it's faster than a kart on straights and in high speed corners, but it carries a lot of inertia, too. Comments about shifters having "only" 40 hp totally miss that point.
2. FAs can be affordable to purchase, but tend to be white elephants to maintain. That's because every component is highly stressed and tweaked to near its absolute limit. In more than a dozen years of owning and racing FAs, I have spent
at least $500 per hour of track time, exclusive of consumables (tires, wings , gas...) and travel costs and entry fees. The engine needs a $6000-$8000 rebuild every 10-12 hours, and the gearboxes consume dogs and gears like there's no tomorrow.
Older designs like the Swift BD-4 and the Ralt RT-4 are even worse than the later cars from those same companies. The DB-4 suffers from perennial tub delamination issues, and lots of parts for both cars are getting hard to find. Besides, they are popular as vintage cars where they can be pampered like the princesses they are.
For your Atlantic bang-for-the-buck award, the Swift 008 is hard to beat. They are not quite as quick as a Ralt RT-41 or a Swift 014, though, which explains their lower asking prices. Just be aware that if you buy one, you or someone
will become an NST gearbox expert if you are to keep it on track.
3. An FC-Zetec or an FE is 90% of the fun and outright performance of an FA for a fraction of the per-mile cost. To tell the truth, if I were doing it today I would get one of those two cars, with a possible nod to the FC-Zetec. They lack the high-winding muscle of an FA, but since they have more torque and are 100 lbs lighter, they come close to making up the difference. You can also bang on them weekend after weekend, just pouring gas into the damned thing. It's not that they're "low maintenance"...they use the same components as Atlantics, they're just lower stressed. That is...except for their geaboxes if you are thinking of a power boost, which have a practical hp limit of about 190 hp.
4. An honorable mention should go to the Star Pro Mazda, which has a carbon Atlantic tub, 6-sp sequential Hewland and a 6-port Renesis engine, though you won't find one in that price range.
Hope that helps.
Stan