Hmmm...
I wondered if there were a such certified aiplane (I'm sure there is) that would allow a single-engine private pilot to get mutiple endorsements at once
It would be a high-performance complex taildragger.
Then later it could be retrofitted with floats.
Emphasis mine... Let's go through these:
Wouldn't an Amphib 185 fit the bill all in one shot?
300hp, Flaps, CS Prop, Floats, Retract Gear. . .
All in one shot? Nope. It's not complex unless it's on the floats, and it's not tailwheel unless it's off the floats.
Oooh, great pick, somewhat obscure... But it won't work: The Meyers 145 doesn't have a >200hp engine so it's not high performance, the Meyers 200 is tri-gear so a no-go on the tailwheel, and same with the Meyers Interceptor 400.
BTW, it's really too bad that the Meyers Interceptor 400 didn't make it. It got certified, but only two were ever produced, one of which was pretty well trashed when it went down in shallow water off the California coast after the pilot left a fuel cap off in preflight and experienced fuel starvation.
However, what an awesome hot-rod a 4-seat, 400shp pressurized turboprop would be!

240 knots at 31 gph, 1500 fpm climb at gross.
The Republic Seabee fits these criteria perfectly.
We have a winner - And you don't even have to add floats!
Great airplane, but not tailwheel.
There is at least one such plane - it is called the DC-3:
Not single-engine.
If you just want something to get your complex/high performance/tailwheel endorsement in one stop shopping without the floats, that can be done in a T-6/SNJ:
Winner #2! Expensive, but makes up for it in style points.
Not tailwheel - None of the Lakes are.
Forgot to add the Beech 18 meets the requirements as well.
Except the single-engine requirement.
Again, big on style points, but has the same issue as the 185: It's not complex unless it's on floats, and it's not tailwheel unless it's on wheels.
Except it's multi-engine.
So, so far, the only planes which meet ALL the requirements are the SeaBee and the AT-6. To that, I'll add that a "Super Swift" with a Conti IO-360 at 210hp also does the trick - HP, Complex, Tailwheel, single.
You can't add floats to the AT-6 or the Super Swift, though. So, I guess the SeaBee is the true winner!