I have 3 Pluses. One is a 128k Mac converted using Apple's upgrade kit (it has a signature case!), the other is a "real" beige Mac Plus and the third is a Platinum Plus. I got them all for free as I found them being thrown out at different times in different places and they all still work. Other than the retro-cool factor, they really aren't all that useful. Most people just turn them into Macquariums. Even if you find a CPU and RAM upgrade for them and get an external hard drive, you still aren't running anything newer than 7.5.5 on it because the ROMS aren't 32-bit clean, which is a requirement in 7.6 and above. Finding system disks, parts, and software gets harder every year and they can't use modern peripherals with USB, Firewire or Wi-Fi connections. I'm not certain you can even use Ethernet with them. You might be able to find what you need to surf the internet with a modem or connecting remotely through a newer Mac or PC, but it's going to be hard with the tiny b&w screen and you won't be able to use the latest media players so a lot of content won't be playable and the connection will be slow.
You're going to have to hit ebay for mice and keyboards. The earlier keyboards without the keypad will work with a Plus if you can't get a proper one. That's what I use on mine. Keep in mind that Apple used several keyboard/mouse connectors over the years that are unique from PC mice and keyboards. Be sure you know what you are buying as ADB and USB keyboards and mice will not work.
So, yeah, having a working Plus is cool if you don't plan on going too crazy with them, otherwise it's going to be a money pit to max it out and even then you won't be able to do much with it.