Moving funds from MultiBit 0.5.18 into new Software (Electrum)
You can use it. Electrum, nowadays, is as popular as Multibit used to be, so you can assume that you will always be able to recover your coins.
To be safer, you can export the private keys from Electrum and save them in an encrypted archive such as ZIP or 7zip. (But you need to do this every time you make a transaction)
What I would actually do would be to create a new wallet in Electrum, but this time, of type HD wallet. Then you can write down just a few words (12 or 18 ?) and be able to physically backup your coins. This would have the advantage of not having to update the backup as all your private keys would be deterministic childs of your master key (=those words)