i want to get a BEAD tuning but i have standard tuning shud i by a set of strings for a 5 string bass and use that or just tune it down?
Switching between the two would pretty much require a string change and definately require a fair amount of re-adjustment. Dropping a standard set of strings down 2 1/2 steps will be extrememly floppy, whilst you may well snap a few strings, and put a shedload of tension on the neck by tuning the lowest 4 strings of a 5 string set up 2 1/2 steps. If you string your instrument correctly, a slightly larger nut slot shouldn't be an issue if you decide to go back to a regular 4 string set at a later time. New nuts aren't that expensive either, though.

As for what strings- that's completely up to you.