it is an extremely odd question
how does a car feel like driving and you have already decided how to modify it ?
i would expect to ask for how does an stock e9 drives like but a modification will be another thing and every mod is different
to me one of the best answers is, provided slow and fast are relative words:
it's more fun to drive a slow car fast than a fast car slow... especially one as beautiful as an E9.
my 2 cents,
to enjoy an e9, to understand this car or anyother car you should drive it stock first, this way you will understand it, you will assimilate its place in its moment, you will have to drive other cars from the same era, and tell the differences, and that will be valid for your own experience
a good advice, but very difficult to fulfill is to drive other e9, i.e. i will not leave you drive mine (i am safe anyway you should need to cross the pond first to attemp such a "blasphemy")
i feel that is very relevant your country and your age, if you have had the chance to drive the cars that were available back in the seventies you will have a better starting point but in any case driving an e9 will probably be quite an experience if you keep your senses ready and "open"
generally speaking do not trust anyone saying that his or her e9 drives wonderfully, like a blast, a spaceship, or like a modern car, without any issues, and so on,...first of all those words mean different things to different people, and then every e9 has some hidden rust and some hidden issues, very few people have an e9 that is 100% perfect, and even in that case another e9 can beat them in specific issues like a better iddle, or a smoother transition, or maximum power, or better handling
moreover some of the e9s out there have been heavily modified and you will not know where you are
so your background is relevant, and your actual coupe is relevant too, not all the coupes are the same after 40+ years
generally speaking do not expect a fast car, fast in the sense of power/weight ratio, but this largely depends on your experience with other cars and in the way you drive too, it also depends if the car you use for comparison has been fitted with a ridiculous M6 engine
i can not compare with that 325is so i wont be of a lot of help to you, a friend has an stock e21 it seems more agile but that is my feeling
as you have already started some route on the transformation of the car i will not discuss on this
my experience,
i was able to drive some of that old cars (from 1969 till 1976) in the eighties, small, underpowered, and very basic cars, one thing i remember well is that they behave like boats when you steer corner to corner
my first contact with my e9 was exactly that, it was an elegant boat, underpowered, irresponsive, a lazy big boat
when i repaired the engine, steering, brakes, suspension (all kept stock) the lazy boat started to be a fast boat , but at the same time a nice boat, very safe cornering and no drifting, perceived safety was bad, but real safety proved to be good
then i went the route of CN springs and shock absorbers, and everything became less boaty but less safe as clear drifting appeared when cornering, the rest of the behaviour was better, but drift was not in my preferred vocabulary
finally i replaced the 17mm front sway bar with the 23mm one from an e12 and everything went just perfect to my taste
the rest of the setup is stock, m30b30, zeniths, 4speed, 14" rims with 205/70, only introducing the TDZ ignition to help the stock dizzy and keep the points lasting a bit longer, i use a 100 scheel drivers seat which i feel is an important part of my equation
two days ago an e46 M3 wanted some fight and i said no thank you, it is good to know where you are, i never drive against others but i love to overtake for the joy of doing it, i always help others that are faster than me, i prefer an empty road than a row of dangerous overtakes so i wake up very early, i drive the small curvy and hilly roads and i enjoy every km, i do not go to shows or gatherings, i have better coffee at home, i do not drive it in highways, i like to try those water springs in the mountains and go to buy fish directly from the fishermen boats in the harbours, i have a bunch of close friends with old bmws and vws and we meet for breakfast or lunch now and then and we talk some gasoline
i love to keep the car in top condition through frequent maintenance and fine tuning of the engine, i do not let others do this for me
every story is different, your question is nice, but to me it would have made more sense before deciding the route of your project
IMHO, many people would have loved what the real stock e9 would have been able to provide them, but they never gave/had the chance...
good luck