It might be helpful to start by evaluating eCommerce software to see what's going to work best for you, and then to either select a template or hire a designer familiar with the software, which should help move things along more smoothly for you.
I personally LOVE Pinnacle Cart. It's pricey at $800/license, but resellers get very good rates, and you may be able to get a license for less than $200. It's a VERY worthwhile investment.
Miva Cart is generally well-received. Magento can be very server-intensive, and I'm not a big fan. Whatever you do, please do not use OSCommerce. I've seen too many good people burned by that piece of garbage.
Demo different pieces of software and see how they function. Many double as a decent content management system as well (Pinnacle does this), so you can build static pages around your shopping cart too.
Once you have a shortlist of shopping carts, then you can worry about how to make them pretty
If you do not have technical experience then better hire someone to do the installation of the shop or buy a commercial ecommerce that has support. Buying a template will not do the job for you. Ecommerce websites need some professional customizations and tests in order to not have any issues with your customers.
I also agree magento is resource hungry but its maybe the shopping cart with the most features.
But if you really have a basic store i recommend Opencart just because i find very easy to use and fast.
I totally feel your dilemma. I have generally negative view on template websites. Here is something you also need to consider: you need to be able to place your keyword and change certain functionality. Can you do that with the template sites. The template sites have to offer that kind of flexibility so that you can promote your site through SEO at least.
As part of a firm that specializes in ecommerce, as well as an user of many carts I would recommend that:
1) You avoid zencart, oscommerce and anything based off of them at all costs. They should have died 6 years ago, but it seems somebody is keeping the zombie alive.
2) Avoid Magento if you don't want to spend thousands on support and customization. Magento has a horribly over-complex code base for logic and templates.
3) Look for paid carts. Pinnaclecart and Xcart would be at the top of my list. Both are easy for customizing the template, as they use Smarty and not PHP.
4) Scrap the idea of buying a premade template. Customers are savvy enough to understand when a company buys a template vs getting one specifically for their company. Hire a real design and development firm to get what you need. They will also be able to assist in logic and functionality changes.
5) No one should be able to give you a company that will design a site for you until you know what you're using. The company you hire should be familiar with the software and not just coding.
6) 'Budget' is not a number. I've had some clients call 'budget' $200 and others call it $6000. If you want something custom for under $1800 and have it be of decent quality, then I wouldnt count on it.