When I come to sell mine, if a potential buyer tried to talk me down on price because I replaced the shoddy OE rubber parts with better & longer lasting items, I'd kick their arse all the way back to where they came from & refuse to sell it to them
For things like rubbers, it's pretty sensible to fit better ones like powerflex as they're better & longer lasting, would you fit a set of 20+ year old Pirelli P4000 tyres just to stay OE like on 0001?
The rear beam is easy enough to lower & raise it at a later date so don't worry about having to swap it. If lowering & adding coilies makes it better to drive for you, to hell with the purists & get it done! Keeping the OE stuff is a sensible idea though as things like that will help the car retain value to the purists further down the line so to speak