What is the best way to provide clients with an accurate quote for Waterfall web development projects?

Yes. https://docs.woothemes.com/document/configuring-woocommerce-settings/

I consult with clients who are looking for the right outsourcing situation all the time, and the best way to get started is to figure out what kind of outsourcing partner will be the best 'fit' for your needs. After that, it's easy to find out where that type of outsourcing partner/contractor/freelance hangs out and pick a winner. Try writing up some notes about things like: 1) You mentioned you had a small budget - how small? If your budget is $1500 your options are going to be very different than if it's $25,000. Both of those could be considered 'small budgets' in the right context, so try to put a number out there. 2) You also mentioned that you wanted 'good quality, clean code'. What does this mean and are you willing to pay for that? If you are building an MVP and are really short on funds, you might have to use a less senior developer/team and take the risk of getting sloppier code - but sometimes with an MVP that's ok! If you are building a production app, that's a different story. 3) What is your technical and management expertise? Can you create specifications and manage a developer on your own (not easy to do)? If so, you can save money by using a freelancer, etc. If you are very non-technical and will need PM support, you might consider a small group or agency. 4) What does 'website development' involve, in your vision? Design? Copywriting? QA? Server migration and admin? ui/ux? 5) What is your tolerance for going offshore? You will hear lots of people saying that offshore teams all suck, and lots of people saying that they did it and it went fine. Like most software projects, there is a high failure rate but lots of success with offshore. You can save a ton of money, but you can also take a bath so you need to consider whether you are up to the challenge or not - going offshore takes much more patience, attention, documentation, etc. but it can work. 6) What are the skills/tech that you will need? It's best to get a team that is great at the skills you need. If you want a ruby site, hire a ruby shop. If you want a augmented reality site, find a team that is great at that. Watch out for the generalists :) By thinking through questions like the above, you can change your statement from "Where do you go for outsourcing website development" to something like this: "I am looking for an outsourcing partner who would be interested in a 15k-20k project that will be build in python. This application will be evolved into our production app so the quality must be good. I am a good project manager and will work with the developer on the requirements/spec side, and help test. We will need application/db design and development, server administration, and technical support but all design, content, copy, ui/ux will be provided. We will consider offshore teams but you must have excellent spoken english." When you expand your 'what we're looking for' paragraph, the whole search for outsourcing partner gets much easier! If you like, feel free to give me a call and share your 'vendor profile' with me and I can help point you in the right direction.

Checkout is a high-quality WordPress theme by Array built for marketplaces of any kind. https://arraythemes.com/themes/checkout-wordpress-theme/ Let me know if you have any questions about implementation, and I'd be happy to setup a call.

Well first off I wouldn't recommend trying to power your client's database with the standard WordPress database instance. They can use WordPress for the website all they like. But with millions of records like that, you really don't want to move those data in with WordPress. For that matter, you're probably gonna want a dedicated database server to answer those questions, if the client doesn't already have one. You might also need to investigate something like Elastic Search, depending on what your search requirements are and what sort of database they are using (pay close attention to whether or not it provides full text search ability). Beyond that, and somewhat obviously, you're also gonna want SSL on the website and to make sure you're up to date on federal and state privacy requirements for data like this, as well as best practices for securely transferring data over the web (port configuration, ssh keys, etc). The privacy concerns would raise a flag for me. If those data have anything personally identifiable in them - which they very probably do - then you're really gonna want to make sure you do your research on security of personally identifiable information. Government specific privacy /security /auditability requirements may also apply, so double check that too.

Most of my friends use www.wordpress.org and http://member.wishlistproducts.com/ to create their membership sites. Hope that helps.