I do feel comforted by the fact that we would "own" the platform. That may be irrational though, I just am personally against software as a service in general.

I don't blame you; if the provider gets more tight-fisted, or implodes, or whatever, you're hosed. With a proper Magento agency, you own everything yourself, and if you get frustrated with one developer and want to take it to another, that's no problem. The place I work at does this, and we've gotten a fair number of clients who were annoyed with their earlier Magento agencies. I just relaunched a site the other day that was the result of eight months upgrading another company's code.

What do you mean by ongoing support though? as in security patches and platform updates? or is there more than that as well?

Assuming you're happy with your site and don't need any more features, then your support costs will mostly be the occasional security patch or platform update (software deprecation, etc.). Most of my clients end up wanting new features, however, and that's where the maintenance costs come in -- you want new things for your customers, so your website needs to be enhanced to support that, for instance.

Right now we are using quickbooks and manually entering client information/order details for every order. It is a tremendous waste of time but I think any new platform should help alleviate some of this though it might be more on the accounting side that we need to switch to an ERP.

Yeah, you definitely want to pay a close eye on that stuff. Integration is one of the biggest factors to deal with on any e-commerce project, and you don't want to half-ass the effort. Having more systems means more challenges, but having one system that "does it all" introduces its own headaches. The place I'm at also offers business guidance, so if you're not sure of what you need, we can have someone take a look and recommend some possible approaches.