Hi Gavin, welcome to the forum.
I have taken a look at the examples you have posted and have a good idea of what you are looking for. Implementing a grid similar to the first example using a system like Magento is quite difficult due to the way they manage products. But the second example would be fairly easy using configurable options, allowing a customer to select various options to 'build' a product and be given a price for that.
Magento is fairly easy to work with but I will warn you that it doesn't run well in a shared hosting environment. The memory requirements are quite high and most shared environments limit the amount of memory available to scripts which can cause all kinds of problems. It should run fine on a cheap VPS though.
If you are planning on offering a lot of products and want a fully fledged shopping cart then Magento is a good choice, but if you are only offering a couple of different products then it is probably overkill. A fairly simple PHP page that generates the necessary HTML for each of the product options and then passes the customers choice on to a payment gateway (either Paypal if you are staying simple or something like ProtX if you want more flexibility).
The way this works is by allowing the customer to choose their options and then generatign the code to set the right options for the payment gateway.
This is the way we create our special offer pages for Industry Square, it keeps it nice and simple without the overhead of a full shopping cart system. Take a look at our
'Switch your hosting' offer page for an example.
If you haven't used PHP before or aren't familiar with integrating into a payment gateway then it may be better to have someone else knock up the payment pages for you so you can just drop them into your site.
If you want more information on how this would work then feel free to get in touch with me on
[email protected] and we can work out exactly what you would need and how long it would likely take to put together.