Most door companies stock what you are looking for.

I say go for it as your door is the window to your home. My advice would be to consider the colour of the flecks in your brick to make sure your chosen colour works with it and not to forget about the colour of your roof and window frames. For example I would have loved a red door but it would have looked ghastly with the green roof on my 25 year-old house. Instead I did black with dark grey window frames (which truly just look black) which works to pick up the black in the brick. I added my big pot plants with green foliage and still got a touch of red in the form of an old chair I picked up from an op shop years ago. The entrance is welcoming, attractive and reflects what is waiting inside. Have fun and if your chosen colour isn't quite right do what I did and find a way of incorporating it into the scheme somehow.

if the door is solid wood (you can usually tell because it will be very heavy) and the putty and or timber mouldings supporting the glass is carefully repaired and the hinges and door handles are replaces and sealed all around and the door is painted on all SIX sides with a good quality paint/varnish it should be OK but no door will withstand the full blast of hot sun and rain so hope there is a porch or wide eaves for protection...a light weight hollow door will probably only last long enough to save up for a new door, so hope the retro door works for you...btw make sure the door is resized and installed to suit all the extra coats of paint and have the security door made last as old style door thicknesses can affect the installation requirements and costs of the security door

You say you want stain ideas for garage door. This seems unusual, because usually garage doors are not painted, they come in a colour made in a factory. Do you just mean colours. I would suggest Dulux beige royal for render. Colorbond Evening haze for garage door. Then a cedar stain for front door. Or the front door could be any colour really. Maybe Evening haze to match garage door, or even a red front door.