I would think changing this amount you'd also add PAS as it might not even be possible to retain the standard steering. All of which would mean unlikely to retain its identity if you look at the guidance on the .gov site
You could still build it and IVA it however. The guidance doesn't stop you creating such a vehicle. It just means it wouldn't be historic. Ultimately it might be easier to buy a 110 and just swap the engine and make it look like a 109 with a Series front panel and bonnet. That would be a more legal way of doing it. Although I'd put a penny to the pound you are intending not to pay road tax or go for an MoT, hence wanting to use a 109.
Be aware the regs on MoT/VED exemption are also slightly different from either the Rebuilt or Radically Altered Regs you'd need to adhere too.
Aware of the points system
Not intending to build or drive an unsafe vehicle
Interesting idea regarding a defender with series body. Now that may be an option and means off the shelf conversion components can be sourced. Noted, thanks.
MOT / tax exemption / historic status is at the bottom of the list of desires tbh. Already have a defender 200tdi so plan is to go for something else this time around. Looked for a series 1 before settled for the defender though at the time was unable to find one to sufficient standard I would have been confident to use as a daily driver. This was around a decade ago.
The actual lure to series is due to frustrations whilst working on the defender
Plastic trim covering areas need access to cause sheer frustration. Zero enjoyment or tolerance for restrictions to maintenance.
Whereas series are components laid bare with instant access for the tools. Bliss.
Just an example - washer jet replacement on the defender requiring dash out
Cut two holes for my hands instead. I accept to most people this reasoning will appear silly but to those that lay on the drive in all weathers and seasons to maintain as they go...I'm sure there's at least one other person that is nodding.
Let's not get started on modern vehicles. Eg the X5 was ten hours for a full dash removal to replace the fan blower. Sigh.
PAS not interested in. Standard steering can remain if can keep the axles, albeit heavy duty components may be required after some testing.....pinion, flanges, shafts etc. trial and error.