If it were more of a dedicated 'track car' I would tend to shy away from the whole Gr.5 idea as all it does is just make things harder and create more work. Costs on regular consumables start to get out of hand when things get 'bigger'... just tires alone will be more than double in cost over something more 'normal'.
I think its a good idea that you are more interested in the building aspect rather than the track/performance side of it so then you can focus more on the visual things about it... Since you have already chosen a motor platform for it, I guess I won't suggest much more there... aside from 'Think outside the box' when it comes to suspension and driveline. Don't be obligated to stay with E21-based parts...
You mention flying over a metal fabricator. Are you doing the bodywork in metal?! Trust me, you'll definitely want to do it in fiberglass. Cheaper, easier to work with, and relatively easy to get copies made in case you will need extras (and for large front spoilers, thats definitely going to be needed). The actual Gr.5 cars had not much metal left in them... Check out the pic of the Junior Team car I will attach at the end of this post to get an idea of the fenders (or lack thereof). The American-run 320s used by McLaren are even less of a 320 underneath, with some of the cars not even having a front clip/floor/transmission tunnel. Maybe you can get some ideas there as to my 'Think outside the box' hint.
I must again ask you... what are you doing for wheels? Wheels will make or break the look you are going for, have you looked into it? Another thing I may suggest is ask your racing buddies to look into available tires you plan to run and base a wheel size/selection around that. When I looked into it for writing up a list/spreadsheet of everything it would take to make a visually identical replica, the total of rims (getting them casted, machining, finding used wheel sections, and adapters/nuts) totalled to over 8k... Kind of a lot for wheels that just keep the look legit, but not even to the same specs or weight as the originals.