DSF

It depends what kind of SSL certificate we are talking about? For simple websites/small e-commerce you can buy even such simple certs like COMODO PositiveSSL for £3.41/year or RapidSSL for £4.56/year. Of course I recommend something better.
Business certificates like GeoTrust True BusinessID Multi-Domain with EV cost £214.78/year. And corporate certificates cost way more, even thousands of pounds, like Symantec Wildcard SSL for £996.93/year or Secure Site Pro Wildcard for £2,562.15/year
If you need a good SSL and one you can afford, go for GeoTrust like GeoTrust True BusinessID for £36.25/year or GeoTrust True BusinessID with EV for £83.45/year.
Installation of SSL itself is very simple. I do understand that some guy value his work, but seriously... £600? For me it's a scum. Today, almost all hosting providers offer possibility of self-installation of the SSL certificates. Even if you pay them to do the job for you, it won't be ridiculous £600.

Your host should really set this up for you. I've had 3 different hosts in the last couple of years (different website) and they have all been happy to set this up. I even managed to get an SSL cert' from godaddy for less than a £tenner (some special offer). I think one of those hosts may have charged around $25 to set up SSL. My latest site runs entirely on SSL

I don't know anything about Opencart but I would think your payment handler should ensure that you are not storing any CC numbers or other sensitive data.
I use Wordpress with Woocommerce - Paypal & Stripe payment gateways.

I would definitely not pay £600 UNLESS your trying to do a complex transfer and wizardry with your URL structure to prevent Search Engines having to re-index your new site URLs. If so - I wouldn't bother.

If this is a new site then I'd set the whole thing up on SSL (Should be cheap and easy) - If it's an existing site then just SSL the sensitive pages, Again - Your host should do this for you.