Unknown K, $400 is way too much for a 512Ke. I rarely see those even get close to $100. I haven't priced these cards lately on eBay, but assuming they are quite rare then maybe. But I certainly wouldn't use it.
As for which is better, a Plus or an SE, I suppose that is all subjective, based on what you want. From a hardware standpoint, a late model Plus will have the same robust components of an SE and run all the same software. My 1990 Plus is still going strong without ever having had a single repair. And it doesn't have a fan, so it operates in blissful silence. Since I am long past needing a Compact Mac for speed, the marginal differences between those two Macs is of no relevance to me. I use a ZIP drive with it, which is ample and silent, so no need for an internal hard drive. Since expansion cards are all but non-existant for the SE (and were at the time as well, not to mention expensive), that is of little use. Now it does have ADB which could be important to some. But that is not enough for me to have to live with what I consider one of the ugliest of the Compacts. The compact was never a business machine (it was the computer for the rest of us), so putting the Snow White corporate face of Apple on it was just a dumb move. They should have just left those dust attracting lines off of it. So it comes down really to what looks best and what serves best for the user.
The only reason I ever encourage anyone to buy an SE over a Plus is if they get an SE FDHD to maintain compatibility with modern computer floppy drives and complete backward compatibility with their stock 128K or 512K.