The best thing to do first, is to buy one of the good guides to the entire walk, then you will know what to look out for on your journey.
Its also worth contacting the Offas Dyke Society.
They used to be located in the old school in Knighton, and they have an extensive acommodation list and nearby businesses that sell supplies.
You may be camping out, or using local b&bs, but either way, the information provided by the Offas Dyke association is more than useful for a first timer attempting the traverse.
I found that the middle sections, near Craven Arms and Clun, to be the most impressive, but their also the remotest sections as well.
Probably the best times to attempt it, would be in early summer, when you have more daylight, and hopefully more settled weather, but over the years, living mainly in South Wales, ive visited over half the entire route, and it is a stunningly beautiful Long Distance Walk.
At around 177 miles long, its a nice distance, but a lot of it traverses very remote countryside, sometimes along way from habitation.
My fondest memory, was getting my car stuck in very deep mud adjacent to the Hatterall Hill section in Mid Wales, back in 1983.
A kindly farmer pulled me out, otherwise i do not know what i would have done, i was stuck fast.