Tell your friend to join the 8-fest.com. Thats where all the E31 owners hangout.
There is also this forum: http://www.bmwe31.com for modification information.
In terms of modding an 8, you'll find few things as well as things being quite expensive. The car was designed as a LUXURY coupe with a sporty body not a sporty coupe with a sporty nature. Also, being somewhat limited in production, tuners are less likely to develope parts to a consumer that most likely will not modify their cars.
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