Rob,
At your weight and altitude the 503 will provide good performance. If you will watch some of Rotorhead's videos you will see that he was able to do incredible things with a light Air Command with a Rotax 447 engine. (much less power than a 503) Having less power will simply require you to learn to be a better manager of your power. Many people feel there is less chance of you getting yourself in trouble as a new pilot, if you start out with a less powerful Gyro and learn proper power management.
However, most pilots will eventually want to upgrade to a more powerful engine. I subscribe to the theory that you can never have too much power. Now that doesn't mean an inexperienced pilot can not get into trouble with too much power. As humans, we are very good at finding ways to get our selves into trouble. But having plenty of power, applied properly, is a very good position to find your self in.
Having more power than the minimum required to fly, means you will not have to be flying at max power settings all the time. With the 120 HP Yamaha engine Racer has put on his single place, he has never had to use full throttle yet in any of his flying. That means his engine is running at lower rpms (which is good for the longevity of the engine) and he always has a reserve of power available if he ever needs it. Now he is flying out of a high altitude airport so he doesn't really have 120 HP available to him, but he still has around 100 HP available to him and that is more power than he needs to fly, so he is able to throttle back and have more control and more options when it comes to his power needs. I think that is always a good idea. I have yet to hear someone say, "I really put too big an engine on my Gyro, I really don't need that much power and I wish I had stayed with a smaller engine."
If you can afford it (more HP always cost more money, both to acquire and to run) and if it is done right (with a good power to weight ratio) it is always nicer to have more power. But everything comes with a trade off. More power = more cost and more weight. To me, it depends on where your priorities are. For example, in my case I am wanting to fly over mountainous terrain that could mean very limited areas to put down in case of an engine out. So the absolute ultimate engine reliability available is high on my priority list. So I have decided to go with a Rotax 912, 4 stroke engine. It is a very expensive option that many people on the forum simply would not consider for their machines, but for my goals and my priorities, it is the best choice and I have had to find a way to make it happen. Only you can decide what is most important to you. Once you know what you really want, then others with experience can help you decide what options might best meet your goals. Deciding what you really want most, can be a long process or can be arrived at rather quickly, depending on your personality and the urgency you are feeling over the decision. Regardless, enjoy the journey.