Hi,
I have over 20 years experience in programming with more than 15 years in web development and quite some years with mobile apps. I can implement your ideas fast & reasonably priced.
As far as I understood you need some website where "investors" can sign up and pay $5 to get "shareholder" of your upcoming PTC website. They should probably be some kind of counter, saying, that only X more investors can sign up, ...
As a payment processor I would choose PayPal, as it is the best deal for mirco-payments like $5.
The infos will have to be saved in a database for which I would choose mySQL if you have it on your server/webspace.
Please tell me how you want to manage the member infos ? Do you need some kind of admin panel ?
I do the "agile development" approach - meaning, that through the entire process I update you with new versions, as soon as it has some major changes / new functions. You can than check the progress and we can talk about how you like it and what has to be changed.
I would start with a MockUp - setting up the user interface for the app, every button, ... will be there but no real functionality. Then I will make this app alive by adding functionality + database.
For sure I will do some responsive design, which looks good on desktops, tablets & phones.
I will be happy to hear from you soon.
Best regards,
Fabian