will a Before Update event be helpful here
Not if you want this to happen when the form opens. Suggest the Load event of the form. At that point, its data has been loaded whereas in the Open event, it has not so no point trying to do this in the Open event IMHO.
If I knew I was going to end up here at the outset, I would have created one function to accept the combo value as a parameter, and assign that value in 6 cases - form load + 5 button click events. I suppose you already have 5 events with pretty much the same code and will want to continue on that path. So enable the button that matches the combo value in form Load. If it is not a sure thing that the combo will always have a value on form load, then your code should deal with the possibility.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.