Web development

I need my website re-configured. The site (which is an e-commerce site) is built on core php. [login to view URL] and PayPal gateway is integrated on the site. We had some J's conflicts which has been fixed. Now what needs to be done is as follows: Stage 1. 1 We have issues in the session - example if 2 or more customers try to checkout at the same time the sales gets dropped. 2 Sometimes upload files gets duplicated or mix up. Also we need to restrict the customer from clicking add to cart button when the file is still uploading. Also for non allowed file formats and large files we need to integrate Google drive API so that the customer can upload such files there and send us the link. Also some more minor UI changes are required on the upload. 3. Customer and admin are not getting email confirmation when the payment is made thru PayPal. 4. Need to improve the site load [login to view URL] now it shows a score of 65 desktop on gtmetrix and load time of 6+ seconds we need to get it down to 2 sec or less 5. State sale tax need to be fixed, right now it work fine with out coupon, but when user applies any discount coupon the sale tax should be applied to the new discounted amount. 6. Need to fix some on page SEO issues, the site is already ranking on first page of Google for several kw. We may have to change the URLs structured and implement permanent redirects in a way that it should not downgrade our ranking but upgrade it. review aggregator is implemented to the site but it does not show star rating on all organic listings - so that needs to be fixed Google structured data content needs to be implemented correctly. Several minor UI css adjustments needs to be [login to view URL] to replace the font and use a font like open San or other font which should look crisp and very clean and professional.  Stage2: Will be a separate project, which requires several new addition s to the site. Including new home page, mobile responsiveness, catagoe version of the site. Dynamic coupon creation and auto email responses, and several more features which we are still finalizing, so this can be long term project for the right person. Most Important: I know that understanding someone else code is troublesome and trust me this could be the most messy code ever written still the site gets over 2-3 online orders each day- apply only if you are willing to work on it - I know the problem, I need someone who is a part of the solution and not the problem. The site is already live and all fixes and changes will be done on a separate server and once tested and approved will be replaced on the main server.