This is getting a bit confusing....reference to the R80GS Basic
If your current Bike is a R80G/S with the original monolever back end, front forks and G/S 21" wheel...all that has been added is the PD kit.......that could be bought separately, (at the time - with or without the seat), for the paralever R100GS/R80GS.
The R80GS Basic is a paralever Bike with a unique rear sub frame, seat and a tank that is the same as the monolever R80ST and almost the same as the monolever R80G/S except for the filler cap.
Your Bike can never, (well nothing is impossible!), economically be converted to a paralever R80 GS Basic or anything paralever as the paralever and monolever frames are different plus the front end, back end, etc., etc....and as I said the R80GS Basic rear sub frame is unique to that Bike and very difficult to come by.
Now if you have a monolever 1981-1985 R80G/S, (what is the year of registration or frame number?), with the paralever PD kit.....all you need is to remove the PD stuff and buy a R80G/S small tank, (£150), a clamb shell,, (that black thing on Kenny's Bike but they came in a number of colours) = (£70), and a round head light, (£50)........the clamshell was a standard feature on the paralever R80/R100GSs, so you should be able to pick one up with or without the headlight unit.
If you want the big original R80 G/S Paris Dakar tank, (which was later fitted to the Kalahari), this could cost you £500 secondhand, (I have just sold a Kalahari Tank for more than that), ....BMW still sell them new for a more that this price but you then have to buy the filler cap and have it painted, plus decals
Also check your Speedo, the little number above the button as I think that they are different on the paralever and monolever......but I would have to check.
I would think that if you sell your PD kit on Ebay you will get enough to buy the bits you need, (with the small tank), as the plastic PD tank is sort after.
Hope this helps