Our status is - I'm employed with a private company and my wife is self-employed (eBay store) with no employee. We file our taxes as "Married filing jointly". We are both under 50. I contribute to 401K via my employer to max ($18,000). My questions are :-

1) Can my wife open a SEP IRA account for herself ?
2) Will the contribution she makes to SEP IRA be tax deferred ?
3) Whats the limit for her contribution considering I contribute $18000 to 401K and we file tax jointly?
4) Is this worth ?