well there are a lot of 'it depends' in this - but in general, you have to itemize, meaning each resource or type of resource would have its own item. The items in turn will carry the rules for recognition as well as the triggers (could be fulfillment-based also, like booking number of hours)
The forecast rule is the budget-part of this but it is optional so reporting could replace.
Adding a resource at a later stage is not a problem since you can have start/end date on each line - if you do not add it at start you would need to add an order and merge the arrangements (you can keep them separate, merging is mainly if there are multi-year recognitions after the addition.)
Not sure if you were asking if ARM could do it - it can and probably the non-advanced version could as well, the main thing ARM brings is the detachment of invoicing from recognition.