Wow alot of questions I will answer them to the best of my ability.
Service providers put sim locks on there devices they sell so that they can sell you a more expensive device for cheaper if you use there services. Ok well cid locking is done by the device manufature such as HTC, Organge, O2, ect... Ok what it does is keep you from puting another device manufatures rom on your device so that you cant put a 8525 cingular rom on a TYTN t-mobile vario II
Ok so you have to cid unlock the device before you can install another rom. What is a rom you ask hard to explain. Do you know what your CMOS is? Its a chip on your mother board that has your computers information to tell the computer to run its start up like your POST. Ok the chip uses memory calle rom and the memory doest erase when you shut off your device unlike ram does (this is because the rom has a contant power supply called cmos battery) Ok what does this have to do with a phones rom. Ok the phone doesnt use a hard drive or have other major components like a regular pc has. The rom on your ppc contains sets up instructs for the phone called its OS. If you change this rom you can change different things and how the phone performs certain actions. Some service providers locks there wifi so you have to use there data plan to connect to the internet. So if you use a rom from another service provider that doesnt lock out features then you can use more of the phones abilities.
No you do not need a storage card but they are so cheap Idk why you wouldnt want one.
I would suggest VP3G's rom its the most stable and if you go into a store with your phone with this rom and ask questions you wont throw up any flags like having a t-moble flash screen so they will still help you. Also one of the most stable roms and mostly like the new AT$T rom with alot less bloat and more customability.
Olipro is a software developer from XDA-developers He created a hard spl which will allow you to flash roms with unsigned NBH files. Basically it keeps you from bricking your device by bad NBH files. I would suggest donating a few bucks to him if you use his software.
Hard-SPL, Or How to Not Brick Your PDA Ever Again And Fix Bad Blocks! - xda-developers
You dont have to do this, this is just what I do and others to get the most out of your device.