I advise you to start a topic on
http://www.mye28.com
Until ~1982, the E24 6 Series was based on the E12 5 Series platform. Then it switched to E28 5 Series platform. If I recall correctly, 633CSi was built from ca. 1976 to 1984, so you can find it as either E12-based or E28-based.
For about 10 years I owned an E28 535i, and based on my experience I think you will love the E24. However, as with any car model, there are specific technical issues you may want to consider.
mye28.com is a very knowledgeable community and they may give you the best advise regarding what to look for and which are the typical red flags concerning an E24 633CSi.