If you're going to buy a roadrace bike, you should be accepting roadrace bike geometry.
The quickest method to find your framesize is to measure your Leg Length by subtracting Sitting height from standing height.
Take a tape measure or look on the bike manufacturer's website and find 'Stand over height'. This is the height above the ground to the top of the top tube where it meets the head tube.
Your Leg Length should be 40mm more than the standover height. All the other frame angles and dimensions, seat pin and stem will be suitable for your body size ( if you have average proportions for your leg length ).
For a more upright seating position, a stem with an angle will do the trick. You will have to make a guestimate of th enew stem reach as the handlebars should ideally move toward your eyes, not vertically upwards.