Ask yourself what you enjoy most about model railways then ask if you can replicate that in the larger scale.
I am mainly a 4mm modeller, I used to help out on a 7mm layout and thought I was used to the size of it. I picked up some cheap old Lima stock such as a shunter, some wagons and a couple of mark 1 coaches. I was quite shocked a just how large they looked in my space, even though I have roughly the same space as yourself. Pointwork takes up a considerable amount of space too.
It didn't stop me buying a Dapol 08 and some of their bogie steel wagons but I will have to seriously think about what I want to achieve with 7mm. I'll probably settle for a small shunting yard, which is fine as thats what I enjoy, I won't sell of my 4mm stock though.
You mentioned that to you bigger is better with regards to 7mm, that can be debatable, especially when it comes to Heljan. Their Mark 1's have the wrong roof profile, something is off with their air brakes vans and I think I remember the buffers are too close together on their 35t tanks. Locowise, the class 37 whole body side between the doors from the cantrail down is far too low - I'm not a right counter just pointing these well known issues out. Some of Dapols wagons are a bit wonky too. Heljan loco's have also been plagued with spit gears in the past, which I understand are hard to source replacements for.
You also mention that you can't solder, that would be something you'd need to work on, 7mm is not quite an RTR scale yet and a lot of stuff still has to be built from brass and plastic kits, or indeed scratchbuilt.
Saying all that, I really like the bulk of 7mm, it has a heft to it that is hard to convey in smaller scales, and personally, I think it's the only scale where sound 'sounds' right.
Think carefully about jumping in at the deep end, you'll lose a lot of money on selling your 4mm stock, but you'll lose even more trying to shift 7mm stock.
As others have suggested, try a small shunting layout using Scalescenes downloads, to get used the the space needed. You don't have to buy a Dapol 08 and wagons to try it out, just pick up some of the cheap old Lima stuff from eBay.