You have to be realistic with upgrades.
If you can do the work yourself then it opens up a whole world of cheap upgrades that suit a budget gun. If you can't upgrade yourself then they become a money sink.
For someone who is gearbox savy and can do the work themselves.
Prices from AK2M4.
Mosfet basic xcoretech xet304u style - the nuprol uses a microswitch and a basic fet will keep it working for longer. £7.50 + some wire.
Sort out the airseal.
Add an oring airseal nozzle (£4.50)
Grease and shim the gearbox.
New piston o-ring £0.25
Main spring upgrade £5.
Total spend on gearbox £17.25.
Getting a tech to do the above is going to run you at about 2 hours labour + parts + the basic fee. Well over £50 IMO.
For the barrel and hop.
ZCI plastic rotary hop unit £8.50
Laylax prometheus purple bucking and nub £10
ZCI 6.02 inner barrel £20
Total spend £38.50. You should be able to build and install all of these parts yourself. I'd build it as a complete inner barrel kit and leave the original intact.
Getting a tech involved will run you almost to the £100 mark. Doing it yourself £55. So there is a big difference in the upgrades a savy person can do compared to one that needs the tech.
In terms of added value there actually is a little. So a £130 gun second hand without upgrades in good condition will trade at about £75-80, With the upgrades done asking £100-110 would be reasonable provided it shoots well and looks clean.
The question you have to ask yourself is if you can't do the work yourself where do you draw the line. You could price it for a fast sale £60-70, Save the £100 and have £170 that can be added to to move into the midrange gun section... It's not that much more to add to get into all steel AK territory with LCT or into some of the nicer G&G and specna arms models.
https://gunfire.com/en/products/sa-b15-keymod-14-carbine-replica-1152205883.html
Or you keep it as a working spare, get a new gun, and loan out the nuprol to friends so they get a cheap introduction to airsoft, doing no upgrades at all to it.
I linked to a specna arms on gunfire for a reason. Spend €225 and you get free postage. They will install a basic mosfet for free when you buy a gun and mosfet together, they will downgrade to 350fps for free. So you don't have the initial tech costs to get the gun right from the off, It'll just take a little longer to be delivered.