The bunded ones seem a
lot more expensive. Mine will be a long way from any building, but
should I consider a bunded tank for length of life?

A bund is usually a raised wall or similar that will contain an oil spill in
case of a serious leak. AFAIK self bunded tanks have a structure to contain
leaks. Whilst not legally required for domestic tanks they are a good idea
if you can afford the extra cost.