1500 Calories + 10,000 Steps a day.

Is there a particular reason you don't want to use MFP's calorie goal? 10000 steps a day would probably be active in the activity level setting and then you just have to choose a weight loss rate (max. 1.5lbs per week would be my suggestion, at least at the beginning, and then slowing down as you lose weight).

I'd say it doesn't sound like a bad number, certainly to start with. Results will vary for each individual (some people have slower/faster metabolisms than average) so I would suggest trying it for at least one menstrual cycle/month, preferably two. Based on your results you can adapt if necessary: eat more if you're losing too fast (max 1.5lbs per week on average), eat less if you're not losing weight.

You should know that by eating a fixed amount (1500) your weight loss will slow as you lose weight (a smaller body burns less calories).

Also relevant: if you're doing exercise on top of those 10000 steps (or those 10000 steps include exercise like aerobics or running) you'll need to take that into account and eat a bit more, since you don't want to undereat (health risks).

Yes. I say this based on the math. At 5'5" 185 pounds, active at 10k steps per day, you would be in a calorie deficit. I would estimate based on BMR of 1575 and that level of movement that your total daily energy expenditure would be 2200-2500 so a deficit to lose roughly 1.5-2 pounds per week on average, over time.

But the better question: is 10k steps daily on 1500 calories sustainable for you? Can you do that level of activity without soon getting tired of it and stopping it altogether? Can you feel satisfied, energetic and satiated on 1500 calories per day? If so - great. If not, remember this is a long term project. So if being a little less active and/or eating a little more per day allows you long term success, even if weight loss is a little slower, then that is a win.