A good mirrorless camera should easily last you five to ten years of trips, so it makes sense to buy a body you won't outgrow in six months. The goal is to buy once and buy right, rather than settling for an entry-level body you will want to swap out next year.
The secret to snagging a high-end setup without blowing your budget is timing. When brands announce their newest flagships, last year's models drop in price immediately while still giving you nearly all the same performance. Pair that price drop with a clear payoff plan, and a $1500 setup becomes manageable.
Remember that an intro rate is a firm deadline, not free money. If you still have a balance when the 15 months end, the standard interest rate kicks in on whatever is left, which quickly wipes out the savings you built up by buying older gear.