I couldn't agree more about doing rental first or borrowing from your stepson if possible.
Getting a two-tone before waiting to get a UKARA defence is arguably worse in the long term whether you decide you really enjoy it and decide to get another gun or if you decide airsoft is not for you. Two-tone depreciates a lot more than a RIF because it has little demand from those who can buy the latter. Even if you paint over or remove the bright colour from a two-tone (technically illegal but I'm not aware of any prosecutions), it'll still be considered less desirable and fetch less as you try reselling it.
As EDcase alluded too, there are too many cases where we've had to tell people the harsh reality that they'll have to eat up a big loss to shift any two-tone guns they're reselling.
Sellers who swap original furniture (stock, grip, handguard, etc) on their gun for third-party then list the gun for sale without the originals but charge based on depreciation of the complete original gun and the new furniture.
Is it the semi-auto function or the fact that it's CO2 powered that draws you to it in particular or the looks?
If it's only the looks, Golden Eagle does some M870-based tri-shot gas shotguns with a swappable buttstock and the handguard is something you can work with as a lot of the models have M-LOK.
Pointing out the new hammer is helpful as they are known to break but I wonder why the seller thought it was smarter to push some sensationalist tabloid nonsense in the title instead of something useful like "Combat Master" and whether it's the EMG or Jag Precision version or an unlicensed brand using the markings, the latter of which may be to conceal the actual value.
Is it not better to get the gun first to confirm the muzzle energy or is this a used gun that has been chrono tested?
I assume you're talking about the NGRS version of the M4 CQBR and not the GBBR or are you talking about the buffer spring with a GBBR?
What's the context of a "cheaper alternative"? Adjusting your fps or a less expensive gun itself?
You don't need to give War and Peace but it really helps when you're more forthcoming and transparent with details.
https://www.usedairsoft.co.uk/rif-s/polarstar-powered_1/m4-hpa-tippman-and-a-double-bell-daracys-dragon_i83182
Seller has a Dracarys but it's in parts and he just posts a a retail listing from his phone instead of a photo of the parts he's selling.
Does he not think reassembling the gun gives him a better chance of selling or trading it?