You've asked lots of questions and the truth is that there are many aspects that may affect the answer. I'll try to help but I'm not sure if I'll get you even more confused :shifty:
What the browser will see for sure will be the HTML and the CSS. But of course you will have a database in the background so you'll also need a backend programming language. Which one? It depends on what your hosting uses and, most of all, which programming languages your programming team is proficient width (hosting can be changed, after all)
After that, depending on your budget, the features you need, your maintenance costs, etc, etc you may use for other technologies too...
Hard to know. I would ask for at least 2-3 years of proven experience but then, again, a great programmer may not have experience with a particular language. More than years, I'd ask for their proven experience in similar jobs. Of course, the more they know, the better. But some people learn nothing in 10 years, and others learn lots in 1.
The number of programmers will depend on when you want the project delivered and, again, what one programmer can do. You'll likely need a minimum of two, if not more (one for back-end, one for front-end)
A good designer should be enough... but then you may need a UX expert too.
Depends on the size of your team. I don't have enough details to know for sure... but it sounds like it may come handy.
Most of the time, problems come for lack of communication, bad planning and changes of scope.
Let me explain a bit (I don't want to be boring but it may be better to put an example)
Mark wants a site. He knows that he wants to sell light bulbs. Mark knows his market well but not the internet world. He ask for a website and knows that needs a e-commerce platform and that it needs to be easy to use and beautiful and asks for a quote. Because he's not been too specific, the company that he hires (reputable and with experienced) gives him the quote that they think Mark needs. They include all the usual features that any e-commerce site has.
Mark thinks is OK and goes ahead. While the site is being done, Mark talks to a friend. And his friend tells him all about Facebook and how their site is doing so well thanks to Twitter and other social media. His friend is so enthusiastic! Mark feels that he should do the same! He investigates a bit and reading a internet marketing website learns that blogging can boost your site... He didn't know what a blog was but now he knows and he's thinking... "wow, I think I should do this too!"
So Mark runs and calls the company that he hired and thinks that adding these features would be done as easy as saying "Hello". Just adding a FB button and a new link to the site right?.
Maybe, yes, maybe no. Mark is adding new features that were not discussed before. That may mean that the team has to start over (the desing needs to be adapted... the buttons need to go somewhere, maybe there's no space for a new link), the content needs to be re-arranged and so the navigation system to fit the new link(s), the programming team needs to create a new module or adapt an exisiting one...
This has just proved that there was lack of planning (in this case, Mark's didn't have a proper short and medium term marketing plan for the site, not to say his business) and the scope of the project has changed. And the changes may not be cheap, depending on the amount of work that those changes may imply.
After this small crisis, Mark is happy and the project goes on. But then, when he sees the result of the first test, he's not that happy.
Mark: "Haven't I said that I didn't want to re-do my logo? or change the color?"
Company: "We talked about this. Your image needed some improving and I even have your e-mail with the go ahead"
Mark: "No, no, I've never said that"
Company: "Well, we met this day and this other, and we mentioned what we were doing and why and you thought that it was great!"
Mark: "Then I didn't express myself properly or I didn't undertand what you said!"
This proves lacks of communication. Either Mark didn't understant what was going on, or the company didn't listen to Mark or both.
If there's a framework that needs little adapting for the features you need, then you need one. If a framework doesn't help to accelarate the process (and hence lower the cost) of building the site and/or is not easy to adapt the features you need, then scratch code.
Be aware that someone will have to do changes if you need new features. You should plan for the future and you should have a proper plan (this is really a business plan of things that you think you should do for your business in the short, middle and long term and, as we saw, it needs to include markeing including online marketing)
Of course, this is a development system and there are frameworks specifically based on MVC.
Hard to say without knowing the size of the team and their expertise and the features required and needed. I'd would not risk it for less than 4 months... but I suspect that it will take more. There's a lot of validation to do there for sure.
Not sure what you mean with this... But this is why planning is so important as well as knowing the market you're directing to.
Maybe but I can't think for any other