update joomla hikashop website

my site leonidas-fleur.be (webshop hikashop already latest version) runs on joomla 3.4 with themexpert template on php 7.4 and should be upgraded to joomla 4 -> php 8 , working on standard template then because I'll install a new template myself. The main thing is that the site must run under joomla 4 on php8 can someone help me with this and what would be the cost?