Help me spend my money!

Narrow band reads stoich and thats about it. When do you want stoich? Hardly ever...cruise should be a little lean, power should be a little fat. Better than nothing but hardly worth the effort when widebands are under $150. You can use spark plug antifoulers for bungs.

How about under a hundred including a 4.9 sensor? A board member from another DIY board
http://www.14point7.com/products/slc-free.

Look at option C