All you really really need is a laptop, passport and credit/debit cards. But you'll be much more prepared if you take some time to plan ahead and get hold of some of the things listed in the sub-chapters below.
If you need to buy something on the road, then southeast asian cities have you covered. However the process of finding what you need will never be as easy as at home. Also, buying things ahead of time from home saves you from the hassle of buying lower quality fake brands that you don't recognize or trust. Consumer protection laws are non-existent or unenforced, so you're rolling the dice whenever you buy something from a local shop.
I'm going to write a few blog posts about digital nomad equipment and link to them all below: