I would recommend seeing if you cant find a used one through ebay, craigslist etc. to save some money during your weightloss time then go get yourself a new one (or another pre-owned) after you reach your goal weight.

rental until you hit the target weight?

1. Rent one at Speedy Reedy over by Gasworks park. I think they credt you for the rental toward purchase.
2. Play it Again Sports in Woodenville has had wetsuits from time to time
3. Personally, I have only worn a wet suit in local races here once when testing it out before my Ironman. The water really isnt that cold so you could get by with out it, esp for a sprint.

Same situation for me- Wetsuitrental.com- costs about $60 with shipping there and back. $40 rental fee can be applied to the purchase a used rental from there stock. Easy process, can apply up to 2 rentals ($80)to the purchase price.

Well locally here Speedy Reedy is like $35 (if I remember correctly) for 5 days.
Also if you are on the East Side--Everyday Athlete in Kirkland and Issaquah Sea and Ski rents as well. That is what I would do personally unless you are actually swimming consistently in Lk Union or Lk Wa--but even then summer temps are around 75 deg. I usually do my OW swimming in Lk WA with tri shorts and tri top.

+1 to rental. If your anxious for the purchase just do your homework. Usually the sizes fit a range for height and weight. If you pop out of that range after losing 15 lbs maybe just sticking with the rental idea until you lose the weight. If you are at the top of the range in weight and you lose 15 lbs that may just put you at the bottom of the range but in the same size.

I lost at least 10 pounds since I bought my wetsuit, and it still feels fine and fits tight. Jeans, shorts, and khakis are another story.