I've ran an online shop in the past so hopefully I can add some insights and hints to add to your knowledge toolbox and also help answer some of your questions.
>>i am wanting to know if any of you have ever set up a parts store CSS site and would have a template?
You mention you want something like aci so I assume you're talking php but if you're into .net pages, nopcommerce is the way to go. Its a free download and an amazing template you can work off.
>>if there is any crazy point i need to watch out for or buy... like a SSL certification?
Unfortunately, it looks like you're leaning php so I can't be all that much help here.
>>lastly using Paypal do i need to really offer anything else to my customers? plus can you have a UPS Ground calculator before or during Paypal's control?
Depending on what type of template you get, some will have UPS calculator already made for you, some will not. If you want to have a UPS (or USPS or FedEx) calc in your page, you need to set up a (free) account with UPS, FedEx, etc. Then you can tie that to your pages and have it before the Paypal payment screen. I just summed up a whole big chunk into this one big generic paragraph so if you have specific questions, i can elaborate.
Also, food for thought, when I ran my site I noticed that some people were wary of using Paypal. Now, you don't need a paypal account to pay by credit card but when some people see paypal's payment screen, they abandon the purchase. It's multiple things but some of it has to do with it looking like you're a small company. In going with that thought process, it may be good to set up multiple payment options like Paypal, Google checkout and even your own CC payment page (SSL of course). The thing with doing your own CC payment system is to set a merchant account with a bank. I know there's some services out there that let you use their merchant account and they fund you but I'm not really sold on those services. Finally, if you do set up your own CC page, you're responsible for transmission, processing and perhaps even storing CC numbers and such. If anything goes wrong in that process, you're ultimately responsible.
Anyway, I just rambled on about whatever I can think off the top of my head since I'm at work. My site never took off, it did okay but it was worth too much trouble for the money it brought in. I don't care what anyone says, you will NOT get rich unless you put in the work and with 3 very young kids, I just didn't have time or energy to put in the extra work to make it grow. I closed down after a year and half. It turned out to be some good side cash and some very good experience. I'm eventualy planning on doing something like that again in the future when my kids need a bit less attention.