If it has DVI then that is your best option. You can get a VGA to DVI plug and use it with the Dreamcast VGA box/Cord. They are cheap, might cost ya a buck. I have a bunch of them. Make sure to buy the right one as they have a few different forms of DVI. You should also see if you can find out if the DVI input supports 480x640 resolution. I know that the tv itself will indeed support it, but there is a chance that the DVI input may not. I have had tvs with DVI in the past and this method did indeed work on them.
I own a cheap 4k tv and it has pretty much all of the old inputs. Not sure if it has S video, but it does have component, composite, and RF as well as HDMI 2.0, MHL etc.
As for other PAL users? Yeah they are around a good one to ask might be the other moderator mazonemayu
The only VGA to 4k scaler that looks worth while to me is the Kanexpro VGA to 4K Converter. It can upscale a 480x640 signal to 720p, 1080p, and 4k. I cannot find a review for the VGA model but they make a S-Video/Composite version and 4 people gave that one 5 stars. Trouble is this thing cost over $100 on most sites. I did find it for $76 on one site though. The second issue that I have with it is that It would pretty much only be good for my Dreamcast. The last converter that I got has more uses so I am on the fence with buying another one for this price. If you are interested in it and want to read specs here is there web site
http://www.kanexpro.com/item/?id=CON-VGA-HD4K