Re-build my existing website

My eCommerce website is not working well despite having had recent Joomla updates and malware removal. It needs refreshing or rebuilding. We need updating to the latest Joomla, and plug in issues need looking at. My current site has 6 information pages, plus a contact page, checkout/cart/payment, and customer registration/login. I don't need any new design features except maybe a slight change to the colour, just to refresh it. All existing features (customer registration and login, forgotten password, contact us page, checkout/payment gateway integration, automatic order confirmation emails) need to be incorporated and working well with an administration panel that I can use to add/remove stock, edit page content, view and download orders, and notify customers etc. Stock control MUST be 100% watertight with no over-sales of out of stock items (which happens on my current site) We only sell a maximum of 10 different items (in very limited numbers each week) so we do not need options to add hundreds of lines. I need payment gateway integration, notifications for out of stock items, automatic emails when orders are placed etc. I will also need all my existing site content copied over to the new site. My existing email addresses need to be incorporated for order confirmations etc. I will need a holding page for my current website while it is being rebuilt so my customers can see we are offline temporarily. I have a 4 week window to get this done whilst my business is closed over Christmas and need someone who 100% knows what they are doing and can test the site to make sure it is fully working before it goes live. Job can be started on or after 16th December - I would like it mostly finished by 30th December to allow time for testing. It has to be ready and fully tested to go live by 11th January with absolute deadline being 15th January. I do not know the technical jargon to explain exactly what is needed but I can send a link to my existing website which will hopefully show what we need. We are on a tight budget with this, but it is very important the job is done by someone trustworthy who will ensure it is 100% working correctly and provide ongoing support for any tweaks needed. I have no idea how much this should cost so have chosen the lower estimate as it is a small business website.