If you want to upgrade with avios you should check award availability in the upgraded cabin before you buy. Bear in mind there may be significant costs in avios or cash depending on upgrade type.

in terms of paid upgrades, they are never guaranteed to be available, even less to be good value. In a nutshell, ba doesn’t need your (or my) upgrade money or avios. At the moment j and f are routinely full on many routes so I guess the advice would be forget hoping for upgrades, buy what you are happy to fly from the start and never count on decent upgrade options - if there is any that tempts you, just treat it as a bonus.