A fixed strike is the only version where the option does the work you're paying for. Appraisal-at-exercise minus 3 percent transfers all the appreciation risk back to you and leaves you with a discount that a decent negotiator gets in an ordinary purchase.
Run your $425,000 against what you're paying for it. On a 36-month term with, say, a $12,000 option fee, your break-even is roughly $437,000 at exercise, and that's before any rent premium and before closing costs. So the fixed strike is a bet on the house clearing about 6.6 percent total over three years. If your submarket has been flat, the fixed number can lose too. It just loses in a way you can measure at signing, which the appraisal formula doesn't.
The collar warrant describes is the structure that actually gets signed, and if you go that route the fight moves to appraisal mechanics rather than price. Name how the appraiser is selected, what happens when the parties disagree, who pays, and what the tie-break is. A single appraisal chosen by the owner is not a price, it's the owner's price with a stamp on it. Second-appraiser and averaging language is standard and worth insisting on.
One thing your framing skips: at 36 months the risk that matters more than price is whether the option survives events at the owner's end. If he takes out a second mortgage, stops paying the first, gets a lien, or dies, your unrecorded option is a contract claim against a person rather than a claim against the property. Whether you can record a memorandum of option, and what recording gets you, varies by state, so ask a real estate attorney there what protection is actually available before you sign the price terms.
