Hi Ronnie.nsu and welcome to the forum,
Question 1, that's one of those "how long is a piece of string" type questions, impossible to give an exact answer. By your own e-commerce store, I assume you are speaking about your own fully independent web-store, not a multi-seller platform like an Amazon or an eBay and so on. The problem is, it's not just a matter if how much you pay for your web-store, whether you take a template style platform like SaleHoo Stores or have your own custom built. There is much more involved that just getting the web-store online.
That's stage 1, but now you have the problem of how you drive traffic to your web-store. It's not like you're on Amazon anymore with thousands of potential customers walking past your door. Now your at teh very end of an extremely dark alley in a very small town that no one has ever heard of before, so how do customer find you?
Marketing .... and yes, that's not only the difficult part, it's also where you will end up spending your money. Without marketing your own independent web-store, you're just throwing cash down the drain because only buying customers can make you money and if they can't find you, then they can't spend their money with you.
So you need to broaden your thinking on that question, it's not a matter of what the cheapest and best value platform is, it's matter of how much you can afford to spend on marketing and how you are going to spend it to get maximum potential ROI.
Question 2. I'll start this one by saying that there is no "wholesale" price when drop-shipping, those two things are completely different and totally divorced from each other. So to focus on drop-shipping, when you have your own e-commerce web-store, customers will be using your own checkout and their invoice will be generated through that system.
So their invoice will be your site's retail price.
Question 3. So long as you are dealing with a drop-shipper, the package is sent to your customer direct from the supplier without any invoice from that supplier. So the only invoice your customer would get is the one generated from your web-stores checkout at the time of purchase.