Zen Cart Upgrade from v1.5.5. for Bilingual Ecommerce Website

I need an experienced Zen Cart developer to upgrade an existing bilingual ecommerce website currently running Zen Cart v1.5.5. The site uses the same database, products, orders, customers, settings and admin system, but has separate domains for each language: English: https://www.welsh-lovespoons.co.uk/ Welsh: https://www.llwyaucaru.com/ The main reasons for the upgrade are: 1. Security. 2. Restoring PayPal, which had to be removed because the latest PayPal plugin does not work with this Zen Cart version. 3. Reducing fake customer accounts, spam registrations and spam messages through the contact form. We previously had serious problems with this, even with CAPTCHA, so spam prevention is a priority. The site should be upgraded from Zen Cart v1.5.5 to the most suitable current stable version, after checking hosting, PHP, database, plugins, payment modules, template changes and any custom code. The English and Welsh versions must both be fully preserved. The two domains must continue to link correctly to the matching pages in the other language. All current URLs must stay exactly as they are unless I approve a change in writing first. This includes product pages, category pages, information pages and any other indexed URLs. Existing SEO, redirects, rankings, bookmarks and external links must be protected. No existing design, text, Welsh or English content, product data, SEO metadata, page titles, meta descriptions, alt text, images, URLs, redirects or bilingual links should be removed or changed without my written approval. The work must include: 1. Reviewing the current site, server setup, PHP version, database version, plugins, custom code, template, payment modules, shipping modules and bilingual setup before upgrading. 2. Confirming the recommended Zen Cart version before proceeding. 3. Taking a full backup of files and database before starting. 4. Carrying out the upgrade on a test/staging copy first, not directly on the live site. 5. Preserving products, categories, images, customer data, orders, settings, language files, URLs, redirects, SEO metadata, bilingual links and existing content. 6. Checking plugin, module, template and custom-code compatibility before changing the live site. 7. Restoring PayPal with the upgraded Zen Cart version and current payment requirements. 8. Improving protection against fake accounts, spam registrations and spam contact-form submissions. 9. Testing checkout, payments, PayPal, contact forms, customer emails, admin area, mobile/tablet/desktop display, and both language versions. 10. Having a rollback plan in case serious issues occur. 11. Moving the upgraded version live only once tested and approved. 12. Providing a short summary of the work completed. Please include in your bid: 1. Your Zen Cart upgrade experience, especially from older v1.5.x versions. 2. Examples of previous Zen Cart upgrade work. 3. The process you would follow. 4. How you would protect the bilingual two-domain setup. 5. How you would protect existing URLs, redirects and SEO. 6. How you would approach PayPal restoration. 7. What you would recommend for reducing fake accounts and form spam. 8. Whether the site will remain live during the work, apart from any agreed final switchover. 9. Estimated timescale. 10. Whether your bid is fixed price or an estimate. 11. Whether the price includes all taxes, fees or charges you add. 12. Whether fixing upgrade-related issues and post-upgrade support are included. 13. What access details you would need, and whether temporary or restricted access can be used. Please list any possible extra costs, assumptions or exclusions before I agree to proceed. I do not want unexpected charges after the work has started.