There is some good information on the potential hazards of high charging voltage here:
To add my £0.02 worth:
1. With a rectifier only (no battery or regulator) the voltage will vary considerably with outboard revs and load, potentially above 32 VDC. In particular if you switch devices on and off (eg lights), the other devices will see voltage spikes. You may well also see your lights brighten and dim as the voltage varies - not ideal.
2. With a rectifier and battery but no regulator you run the risk of overcharging the battery leading to hazardous gassing.
3. With a rectifier and regulator and no battery, you will get a reasonably stable voltage however outboard regulators tend to rely on the battery to act as a damper and provides a load. It is possible to fry a regulator by running with no load.
4. Cheaper options than the outboard manufacturer's regulator exist, however if you are not particularly experienced with electrics and electronics buying the OB manufacturer's unit will save you time and effort.
5. 2.5mm2 cable should be more than ample over that cable run length.