what should i look out for when bying a 1996 728i bmw and how are they are on petrol

Iv never heard of a 728i. MAYBE your talking about the 528i if that is l own one and they get about 19 mpg and they are amazing sport sedans.

The world is not limited to North America. The E38 7 Series is available with the M52 engine from introduction in 1995 through to 2001.
The M52 is an amazing engine & u can expect as good as 8 l/100km on highway runs. Expect to average around 11l/100 km.
Generic to all E38's
Loosing pixel display in temperature, mileage display.
Power steering hoses leaking
Cup holders failing
Radiator failures. Plastic neck breaking, all years
A/C stepper motors failing.
Original Water pumps with plastic impellers failing
Coolant thermostat failing
Electric cooling fan failing in year 98 & newer
Sunroof wind noise
CD player skipping in models older than 97
Older A/C temperature displays failing
A/C drain lines need re-routing to stop gurgling sound

Good luck. Find a records car or a low km example & u will not be disappointed. Always get a pre-purchase inspection.