There are plenty of free resources out there but you need a fair bit of skill to use them effectively, especially in a complex setup.
You may find the time you spend installing and tweaking your free system is uneconomical unless you're a developer; how much is your time worth?
I would continue to seek a developer - maybe a small outfit, perhaps a freelance coder. Don't accept 'our servers only', if you don't want to. A good outfit will be happy to work with your setup provided it's configured properly.
You should be able to find someone who'll do what you want for a fixed amount but I tend to quote on the basis of scaling because I think it's a sensible way forward these days, especially to risk-averse organisations.
If I were to undertake a development like yours I'd suggest a rate for completion of the basics with a small monthly licence fee (with no tie-in), increasing a bit for each site you have on top, perhaps - or maybe according to traffic, or the amount of ad space you're selling. That way, whatever increases your revenue also increases mine, and if your business shrinks so does your monthly commitment so you're never in for more than you can handle.
I do think there's plenty to be said for the amazing amount of open-source product though.
