I have a Tyros4, but still have an old Yamaha MC600 organ which has 'MOC' ('Melody On Chord'), and I can confirm that it is effectively an older and much more basic version of what is called 'Harmony' on your PSR-EW400, or 'Harmony/Echo' on other Yamaha arrangers such as the MOTL PSR or TOTL Tyros models.
The 'Harmony' function on your EW400 has 26 different types of Harmony, of which the first 4 (Duet, Trio, Block and Country') give MOC, whilst the Tyros1 has 17 types, 13 of which are MOC. I assume that either of these is more than on your Electone!
I also have an old Yamaha DGX-505 which has a similar 'function' based operating system to your EW400, and I have to say that I find it much easier to use the Tyros OS (which has remained broadly similar over the years from the Tyros1 onwards) to make changes to any parameters than I did on the DGX. However I am constantly using my Tyros4, whilst the DGX just gathers dust, so that will have something to do with it!
It is just a case of what you are familiar with, and to get the best out of any new keyboard, you need to read up, at least on the features that interest you. Did you instinctively know what MOC did when you first got your Electone?
I would respectfully suggest that if you can't work out the 'Harmony' function on your EW400, then you will also struggle with a Tyros unless you are prepared to read the relevant parts of the manuals. You may find that if you learn how to use your EW400's functions, you don't actually need another board!
I don't know if £500 is a good price for a Tyros1, but remember that this a fairly old keyboard (marketed from Nov. 2002 to Nov.2005) and so it is not likely to be in pristine electrical condition if it has been used regularly, however 'clean' it looks from the outside!
It has a floppy disk drive, which at this age can be unreliable. It does not have a 'USB to device' socket, so you can't use USB memory sticks to transfer styles etc., although you can connect it's 'USB to host' socket to a computer and transfer files using file transfer software. It may have an (optional) small hard disk which at least increases on board file storage, but you would still need to use the floppy drive or computer to add files to it.
A Tyros2 also has a floppy disk drive, but more importantly, as well as the 'USB to host' socket for connecting to a computer, it also has 2 'USB to device' sockets which can used with USB memory sticks and should be much more reliable than an old floppy disk drive. This would be the earliest Tyros model that I would consider if I wanted a legacy Tyros keyboard!