it would be nice to have mirc highlight spelling errors while i'm inputting text... firefox has this. and so does tweetdeck. :3
This has been suggested many times. However, this would add size and overhead to mIRC that is not needed.
What you could do is use one of the many free spell checking
programs available that would do it not only when you are using
mIRC but also other programs. (including notepad etc)...
One that is free and I have used with some sucess is at
www.tinyspell.com and is called by the same name tinyspell.
This program does not take a lot of resources and I think will
'fit the bill' for what you want.