Answers, hopefully..
I have a laptop, and I do have a decent wifi dongle which was used for my desktop when I didn't have a direct ethernet cord. Its a usb dongle can I use that to pick up free hotspots or do I need a different attena?
--- if using this method you'll be fine, as you are basically repeating an existing signal. What you have sounds like it'll work. (Though there is another, way, get an android 4g & wifi receiver with an Ethernet, charge and plug in to your router)
Also laptops usually come with a wifi adaptor installed internally anyway because they are portable.
1. how many outlet adaptors do I need to purchase for my car to plug devices into?
--- Assuming you use a car, rather than get a battery and bag it up and go inside a shopping mall, 1 for the laptop, 1 for the router.
2. So my laptop is plugged into its own outlet and a router with DD-WRT support is plugged into it own?
--- Your router is now connected via Ethernet to the laptop (or in the suggestion above, the android wifi receiver). DD-WRT will also need chillportal installed.
3. Using the usb dongle how would I send that connection to the router? Then I need to make it unsecured obviously and be able to rename it how would I do this?
Buy a router with USB capability and plug is straight in the side. DD-WRT supports a broad selection of them, look ip their wiki. For the captive portal / naming the network, that's all in the router / DD-WRT.
4. Now where do you host your welcome landing pages on? Do you use a website you already have then just make a hidden page? Secondly how do I input this landing page into the wifi portal so that when people do connect it automatically loads? How do I do this? I have some wordpress sites that im not using much at the moment but I can log into them and make a page and put some offers on it etc but how would I use that same url as the welcome page when people connect to the wifi?
--- they're hosted on your website (you create it as an exception from the allowed sites on the router). Next step, I'm clueless, my router is en route and I'll find out this weekend.
5. As it stands I think I have to buy a new router, dongle possibly, and outlet adapters for my car, is there anything else that I am missing?
--- depending how you do it, either new high powered router, use your laptop and outlet adapters for the car. Or android wifi receiver with Ethernet, ethernet to a tp link captive portal router (outdoor) and buy a high amp 12v battery.