I have an idea for an off shoot of an already successful business that would allow me to scale it quickly and greater than I can now. However, the idea requires the building of a web application/website and I don't have that knowledge. Lots of questions here...
I apologize for the vagueness, I will try to answer any follow-ups that clarify anything. Thanks for reading.
I am a professional developer and I have worked with startups. One thing I would warn against is the notion of some kind of set it and forget it development. Unless what you are looking for is a straightforward web site/application, launching is just the beginning. You'll never get it right on the first try, and you'll need to be able to rely on someone for both maintenance and likely continual changes. After actually using it you'll find bugs, usability flaws, additional features you want, etc. That does not mean you need a technical co-founder, it just means that whoever you get to help you should be a person or company you can rely on to come back to the project whenever you need it. (And don't underestimate how often that will be.)
The chances that you will be dead-on with your website specs or the business model are slim. If you are running a software service, there is more to the technology than just your website code. Increased client size will require code and hardware improvements eventually; unless you spend a lot more of your money up front.
As Jeff said, there are many companies that do exactly that - build websites and applications for other businesses. To determine whether or not they can build what you need, ask to look at some previous projects they've worked on.
A quote on development costs is usually free. If you're going to have to sit down with them for long lengths of time to determine what exactly it is you're trying to accomplish, expect to pay for their time.
If you go with a technnical co-founder, then you'll need to know in advance how to filter out a good partner from a bad one (there's more than chemistry or technical know-how). You will also be giving up equity, but saving yourself the development costs. There is also likely to be more on-going development needed once the project is "finished", so don't be surprised when what you might consider routine maintenance is actually a full-time job.
If you need referals to some companies, get in touch with me directly and I'd be happy to pass on the names of some such businesses I've worked with before.