Hi mumzie,
You’ve probably already seen this but just in case … the list of better options I’m aware of in the Portland area are in post #2 here which includes one of the members of this site which I believe are among the best quality/value in the country.
With higher weights … it’s particularly important to make sure that the materials in your mattress … and particularly in the upper comfort layers which are the weak link of most mattresses … are good quality and durable. This means that I would tend towards 5 lb memory foam (and/or 4 lb gel memory foam if it is one of the more durable types). Lower density memory foams may feel softer and more comfortable in some cases but you would be making a “tradeoff” for less durability with weights over about 200 lbs or so. If you do choose a mattress with lower density memory foam … then thinner layers will have less effect on the overall feel and performance of the mattress when they soften (and they will) than thicker layers. Bear in mind as well that even 5 lb memory foam (and all foam for that matter) will have some initial softening over the first few weeks and will soften more rapidly over time under heavier weights so I would tend towards slightly firmer choices than you may otherwise choose.
Phoenix