Firstly, congratulations on making a great decision. Your bodily and mental health will thank you for it!
I'm close to your height and age at 6'1" and 45yrs.
What size depends on the manufacturer.
For example, the Mongoose Tyax Super hardtail MTB I bought August last year is an Extra Large, with a horizontal top tube length of 620mm. A few weeks ago I bought a used Cannondale Jekyll dual susp. It has the same top tube measurement, but is rated as a Large. With a slightly longer seat post and riser bars it too will a great fit.
I'd suggest you go to zinncycles.comFitIntro.aspxOpen ↗ and follow the guidelines. Being a beginner (again) you will probably want to err on the less "aggressive" side of the measurements Zinn recommends. Bear in mind the longest cranks on stock bikes top out at 175mm.
Being over 100kg you will want to go mid-range price-wise. Stay away from the supermarket type bikes. They will not be strong enough for you, and you will not be able to get informed advice about bike fit, which is crucial to your enjoyment and staying injury-free. Go to your Local Bike Shop and talk to somebody who knows their stuff.
Too cheap a spec will see you generally suffering mechanical unreliability and not enjoying your riding. Dual Susp bikes are roughly twice the price of hardtails for the same component level. If you are going to do mostly road and light trail riding, a hardtail with lockout forks would probably serve you best. Softtails are lot slower and more tiring to ride except on rough trails, and require more maintenance
