I'd be wary of a 2 post lift in 6" of concrete, it might be OK in theory, but that's assuming there are no issues with the concrete at the two relatively small points that they're installed and that the smaller number of anchors have been installed perfectly. Neither assumption you can really test and once you've a car up in the air some of the loads that may be created are going to be fairly large.
A 4 post lift will load the concrete completely differently, there'll be no leverage effect front to back as the opposing posts will effectively cancel each other out. For a four post lift the concrete is taking 4 point loads to stop the lift sinking into the ground and (I presume) tying the four posts together. If the car tries to tip in any direction it'll largely create larger point loads on different posts. With a 2-post any tipping will try to pull one of the posts out. The difference is that the four post lift will pretty much always resist the load by putting the concrete in compression, whereas the 2 post can load the concrete in tension. Concrete is much stronger in compression.
That's not to say a 2 post can't work fine, it obviously can, you just need to be a bit more cautious about it's installation and be mindful of how well you can check that the conditions on the ground match what's required. It wouldn't be a huge job to cut out some of the floor and replace with deeper and reinforced concrete for example. You might want to do that anyway if the base of the posts are larger than the post as getting that recessed into the floor will make working on a small car a little easier.
Personally, considering the number of jobs that you can do on a mini under the floor is fairly small I wouldn't bother with a lift unless you want to be able to lift the car up to create a space that allows you to buy another car to keep underneath. Ditto the ramps, I like the look of them, but you need to be able to suspend a wheel for most jobs. I don't know if you can buy them, but I've seen bespoke ply boxes and ramps made up that are in sections, so you can drive the car on then remove the boxes between the wheels and access most of the car. You could then jack and put the corner you're working on on a stand to allow that box to be removed s well. Ply is strong enough generally, but will collapse if the glue fails so I would probably use steel for the boxes that go under the wheels at least. Actually with steel you could use drop-in bridge sections between the wheel boxes which would be easier to transport and store away.
That said, if you're not going to get the benefit of being able to lift the wheel up to chest height and to be able to easily lower it if you need to get more leverage on something etc, and you want something a bit quicker than messing about with jacks and stands then I'd look at getting a couple of bars made up that go left to right on the front and rear subframes, protrude out from the car a bit and have sit-in bits added for a jack at each end and an axle stand set a bit further in, so you can very quickly lift the car and get it on stands without having to lift a bit each side, find somewhere for the stand that doesn't interfere with the jack etc. Ideally if you could find a jack setup that runs a pair of jacks from one pump that would be even simpler.
