Which eCommerce platform for 500+ products?

I really need some advice on which eCommerce platform to use for my business, I'm very confused at the minute as there are so many options and reviews saying different things!
My on-line store was originally built with Actinic which worked fine and had everything we wanted but looked dated.
An IT company built us a new site using WordPress and we've had no end of problems with it...a year on it's still not what we want on the front end and we're finding it hard to use at the back end as we cannot find where to edit certain things. This company is not much help to us either.
Another company has advised that we ditch WordPress and use Magento

My company has over 500 products, ships UK and internationally, has retail and trade customers so multiple prices for one product, some customers pay VAT some don't, some have a discount some don't.
We need it to interface with Sage.
We are all beginners when it comes to web design.

I've noticed that there's a new eCommerce plug in for WordPress, is it worth us getting this and plodding on with it or are we just asking too much of WordPress and would we be better getting a new platform or do we just need more/different support/help?
If WordPress isn't for us, which one would be?!

Sorry this is long winded but wanted to try and give a full picture!

 

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