I have found the most effective way to pave the way for a new purchase is to:
1. Launch into a month of two of compulsive searches of want-ads. Wild goose chases soften wives up quite effectively.
2. At "random" intervals mention how current equipment is hazardous and how, for example, a ROPS and seatbelt might very well extend our time on the farm. It does your cause no harm if wife's co-worker has had a husband badly chewed up by a poorly-equipped tractor.
3. Make it clear over time that a used machine with similar features costs half the price of a new one.
4. Build flower beds and stone walls where a loader would really help.
5. Cut trees with her there to watch for safety. She'll approve a logging winch very quickly, then operate it for you.
6. Work up the garden and plant it, whether she wants a garden or not. For some reason this tactic is very good for boat purchases.
7. For woodworking tools, plan the project in detail with her. Even install the widget, if necessary. Look at it compulsively for several days, then pronounce that that piece of trim is wrong for that project. She'll agree, and allow the purchase of a 200 dollar shaper cutter to make 8' of trim. Honest.