With your height, you'd be great for a Haflbike sized M. He can find the size, by measuring the distance from the ground to the handlebar - at around 110 cm it should be an M, since the difference in sizes is about 6cm apart.
The springs are considered a consumable and they do get softer with time, if the person rarely used it they should be Ok. Otherwise, you can order a replacement from our part's shop.
It would still be best if you can try it yourself, but note that beginners tend to get hunched over it, while you should keep your back straight. Here you can learn more of what you can expect from the learning curve and process www.halfbikes.com/manual