Hi Ashley,
After check your all parameters i come to know that you need to updated all the functionality of IM except Budget Functionality. We update the budget in Position ID in IM (IM32) which is an integral part of AR because on the basis of Position ID we come to know the details of budget which we assign to AR so system will not allow us to buy pass this functionality of budgeting in IM so we have to go with IMCCP1 then IM54 (Distribute budget to project) Transaction codes to finally update budget in WBS's (CJ30 budget distribute to WBS) of a project. But we should wait for other experts to comment on this issue.
Regards
Shashank Gaur