You know...the nice thing about cross country and, to some extent, dressage is that the bredd of horse is not important. It is their heart that is the most important.
Almost ANY horse can do lower and even middle level of both disciplines. And, luckily, weight is often an overrated problem. I know some very heavy riders who have the lightest seats around. How the weight is used is SO much more important that how much weight there is, IMHO.
Quarter horses, TB (especially TB/WB crosses) Some draft crosses can all be possibilities. Just look around and ride everything you can get on. When you find a "match", then is the time to make a decision.