Andy -
If our configurator is true, the ONE XL regional (UK-only, for example) came with either 512MB or NO internal memory. Neither of those are going to fly for a U.S. map, so you are certainly right that we're talking SD card for this application, regardless of whether it's from TomTom or downloaded to a purchased blank card.
I also believe that the the external storage on that unit was limited in firmware to 1GB, although I could be wrong, and it's 2GB. Makes a big difference as to what map detail is possible.
The trick MAY be finding an old 1GB SD card (or if we're lucky, a 2GB card) that runs at a decent speed (e.g., Class 4, not one of the old Class 2 or 'no class specified' cards). It's easier to find 64GB SDHC than it is to find a faster 1GB or 2GB SD these days.
Guess what we need to verify is whether the ONE XL can deal with a 2GB card (the best he could do for a U.S. map), and where the heck you'd find a Class 4 version of one of those these days. You know anyone on your side of the pond that can confirm one way or another?
One thing for sure - unlike many other TT models, a ONE XL won't let you use internal and SD card at the same time.
The reason I like the idea of a low cost later model is the advantage of a 5" screen and the IQRoutes feature - which the ONE XL never did get - and the ability to hold a lot of Europe at once (v.s. the ONE XL) when the OP gets home.
What are you guys getting for an XXL IQRoutes Europe over there now? I've seen the U.S. model (XXL540TM) for as little as $89 delivered here recently (£57.36) recently. I suspect that the non-TM version that is actually equivalent to the XXL IQRoutes (the XXL540S) can be had for even less.