the UK Web Design Association at click here
Run a search for your area and then make some calls from the list of designers featured. If you explain your minimal requirements you should get some ball-park figures in return, possibly even over the phone. Some designers have a small site startup offer where several hundred pounds gives you a six (ish) page site, feedback/contact form and whatnot. This is probably not the way to go if you want to include some sort of branch to your mail order system.
The trouble is you say you have a limited budget (don't we all) but that you would like your budget to create additional income. It seems reasonable to assume that your expenditure to return ratio expectations are probably quite high. Perhaps this may affect the amount you are considering for your intended budget and maybe a rethink could be in order.
When you mention a point of contact portal that is exactly what most websites can and should be. Your mention of mail order is cause for concern though.
At its simplest, mail order via the web can consist of little more than a series of products on offer on your web pages, each one with a link to a form that allows your visitors to fill in the necessary information to complete their order. The form results are emailed to your for processing. And this is the point where we hit several real issues. Either your customers fill in quantity of goods, name, address, telephone number and so on, and then you contact them to arrange payment for goods ordered, or you start looking into a live e-commerce facility to allow financial transactions in real time. There isn't really much that falls in between the two and if you want to offer a product range on your site then this will send the price of it up, up and away.
If you give a bit mre detail one of us may be able to make a few suggestions so that at least you know some of the questions to ask when you start approaching designers. I strongly suggest that you check out the UKWDA in the first instance though.
I don't ever post details of my own prices in this forum; I can't think of any designer who would be willing to commit to a firm quote based on the tiny amount of information you have provided. It may even be that on completing your requirements analysis a designer may advise against having a site at all. I have done this on several occasions during an initial consultation.
You also need to think about updates. Designers charge for updates and if your site is likely to require regular changes (prices, product(s) etc) then you either pay per update, buy a yearly update package where a fixed amount allows you regular updates for the whole your, or you look at a small scale Content Management System (CMS) that allows you to update your own site content via your web browser. Any good designer can offer a CMS (I offer several with a range of features that cover small, medium and very, very large site demands) but again, the cost factor could well prove impractical. Web programmers also have a living to earn and tend not to give their products out freely.
If you want to ask some specifics then go for it and we'll see if we can help. As I said above though, nobody can really make a stab at offering you a figure without covering all the bases with you, so you either need to provide as much information as possible or start talking to designers. All that will cost is your time.