Hello!
We are creative team of php/js/html/css experts. We have huge experience in creating wp sites with customized themes using jquery, bootstrap, font awesome and other cool stuff. We also create highly customizable, responsive and lightweight themes and plugins you will be happy about. We have large experience in creating e-commerce sites using WordPress + WooCommerce.
Q1: Usually we don't use 3rd party plugins, as most of them are not free, and we can create a custom plugin for exact usecase. We can develop a plugin for group buying including your needs in gradually reducing price. Also we can follow exact workflow you need for the grouping option: products can be on hold until needed group is gathered, or we can activate orders after some sub-group is gathered (for example, if you want to ship products after each 10 orders made, and then the price will be lowered automatically). Please describe the way you want this grouping option to work, and we will propose you some solutions we see.
Q2: Our seeing is that it's always better to use custom-built sites, if the timeline and budget are capable for this. This gives you an ability to run site exactly as you need to. Actually woocommerce itself might be not the best option, as it has lots of features you might not need and on the other hand plugin structure has some limitations about product price changing logic. Currently we don't see any problems for your project with using woocommerce, but would be happy to know more about the ecommerce site you want to built, so we can consult you on the options available.
Regarding marketing for live site you better consult SEO company. Fresh site needs a good marketing campaign with adwords, cross-linking and adding to public catalogs. Also it might need ads on local sites, depending on the area you are targeting, as it might work or not, for this particular location.
We are available and will be happy to help you with the project. Looking forward for further discussion.
Best Regards,
Sergii Savchenko
CEO @ PineDev Studio