I know from owning a mid-90s Micra 3dr and a mid 2000s Mazda3 4dr that despite the former being about 10-15cm narrower, it was no easier for me getting in/out in car parking spaces than the Mazda, given how much longer the Micra's doors were.

I suspect it would be roughly the same experience with the Golf in 3 and 5dr formats. The 3rd Golf mkVII is a nicer looking car (externally, side-on) though, similar to the Leon SC. Both remind me of the shape of the mid 2000s Civic Type R which was also a 3dr car.

I don't know if VW still make a Golf Plus.

They don't, the Golf plus (which was based on the MK5 Golf) was made up to 2014 when it was replaced by the Golf SV. This was essentially the same type of car aimed at the same market, but (in most opinions) better looking and based on the MK7 Golf. It is still in production, but with the march of the SUV in endless, slightly different sizes, you'd have to wonder for how long?. Personally, i'd sooner have an SV than a Golf for the higher seating position, greater practicality (sliding rear seats, bigger boot, etc). I'd also sooner have one over an SUV on account of it not being an SUV!.

was thinking of getting an opel meriva, they seem very high

As for the Meriva, we used to have one, and while it did have good points, it also frustrated with quite a few bad ones bad ones. The rail system running between the front seats might seem like a good idea, but try putting someone in the centre rear and you'll quickly find out why it wasn't. Whoever is unfortunate enough to sit there, will have their legs uncomfortably splayed either side of the rails. The rear 'suicide' doors are good for parents putting a child in the child seat as it puts you in a much more comfortable position. But apart from that, not really any other benefit, unlike sliding doors at the back, where you get the same ease of seating the child, but also once they are a bit older, the added benefit of not having to worry about a door being flung open in a car park hitting the car next to you!. The other minus points i found with the meriva were the heavy clutch combined with a narrow power band (1.7 turbo diesel) and 'too long' gearing. This made the car a nightmare to drive in stop start traffic. And lastly, there was a lot of road noise despite our car not having the bigger wheels of the SE. On the other side, the engine, on the open road, was very strong, and the 6 speed box meant low revs at high speed. the seats were very comfortable and it had a large boot, so were it not for the afore mentioned road noise it could have been a great mile muncher.

I'm still vague about the difference between an MPV and an SUV

About £3k - £5k. Mostly marketing hype, if comparing the latest cars. The MPV is a bit more practical and a bit less stylish, and will likely have lesser powered engine ranges (but not that different, mainly the lack of 'sporty' top end models), thus they come in cheaper.

SUVs of old seemed to me to much more 'agricultrual' (often because the original ones were mainly American) based on converted pick-up trucks rather than today's lot that are now Euroboxes of jacked-up family hatchbacks and superminis, with a bit more boot space.

I suspect the 'traditional' MPVs like the Espace, Galaxy, Mazda5 etc will soon be discarded (at least for a few years) by the vast majority of manufacturers, perhaps with the exception of a few Asian makes selling locally to taxi markets and similarly in some developing nations, where practicality and efficient use of vehicle size, as well as the purchase cost means far more than buying a 'handbag' car to impress your neighbours, work colleages and family.