welcome aboard MrfarNhigh
congratulations on a good choice of motor matey look after it and you can have many years of happy motoring,now to the mods first replace the orange front indicators with nice clear lenses,will cost you about £20-£25 with side repeaters i have a k&n induction kit (about £120-£150 for a good one) and a full stainless steel exhaust system(mine cost £500)fitted to my E34 525i(sounds awesome,trust me you will be noticed though) another mod that looks nice is a nice set of 18 inch 7 series multi spoke alloys(they fit straight on with no messing about with spacers spiggots etc)try tinting the windows as well but be careful with the front ones as i have first hand experience of the implications of illegal tints,hope this helps you out buddy
it's m50What year of 520 is it? Really early ones have m20b20 single cam 12v motors with 120hp, the rest are m50b25 twin cam 24v with 150hp.
The advice varies depending on the motor obv.
What are you looking to acheive? Extra performance? Visual mods? Got any issues with the car, problems you need advice with. Your original post is pretty vague.