How will I know if she is ready for lunch?
TBH, I think that all you can really do is offer it to her! If she eats it, and it doesn't really affect her milk intake at the next feed, then you're good to go

I would wait and see if you can't gradually extend the time between those milk feeds, as as you get to the afternoon feeds you can introduce it and see what happens. If I remember correctly, when Dylan first got lunch his routine looked like (give or take):
7am bf
8ish solids
11am bf
1230ish solids
3pm bf
530 solids
7pm bf and bed
At what age to I try to hold her off at night.
To be very, very honest, I never tried to hold Dylan off at night. But I also never fed him *just* because he woke up... If he woke due to teething pain, I would medicate and shh/pat to help him back to sleep, for example. I am of the mind set that STTN is a milestone that will be reached when lo is ready (just like sitting up, crawling, walking). There are things you can do to encourage it, like teaching independent sleep, ensuring sufficient calories during the day, etc., but that's really all I did. Dylan dropped his last night feed on his own just before he was 1yo.
Having said that, here is a link with ideas on how you can gently discourage night feeds that you might find helpful to help get you there more quickly:
http://www.kellymom.com/bf/weaning/weaning-night.htmlhth!