If you were planning to buy new, can I suggest eBay instead? these are all very popular (i.e common) boxes.
An NDX2 for £3500 or so is a chunky saving versus new. It is better than a naked ND5XS2 - I have both in different systems. If selling the ND5XS2, the net cost would be around £1500.
The alternative source upgrade of adding (for example- an n-DAC to the ND5 XS2 is also an eBay job. Opinions differ on whether that matches/ exceeds a naked NDX2 - it’s fine line, but the net cost of those two source options is pretty similar.
You can find 282, 250DR and HCDR in the same places. A 250DR can probably be found for £2500 and HCDR for rather under £1000. 282s look rare atm, but go for £2000-£3500, depending on age. An SN3 looks to be worth very roughly £2500 now.
If forced to pick, I’d usually say Source First, but the 282/250 is probably a bigger upgrade (even if adding HCDR to the SN3) than naked ND5 XS2 to naked NDX2. That means I lean to option 2 if forced to pick, and that’s a testament to the ND5 XS2 as a bit of a bargain.
On the other hand, given the sums involved and the speakers you have, I reckon you will be probably back soon & doing whichever upgrade you don’t do this time. If that looks plausible to you too, I’d just suggest you hit the auction site, make offers, and accumulate the new boxes in whichever order bargains can be found.
I’d also agree with @Stilts that an NDX2 benefits from an external PS (not a DAC) if that becomes affordable, but then so do just about all Naim boxes so we shouldn’t be surprised. The existence of an upgrade path does not make it mandatory but is also not a bad thing in itself.
There’s wriggle room on all those numbers of course, depending on luck and patience.
Is that any help?