According to the website the bike is intended for a 100 mm travel fork. Staying true to that will mean the least changes in bike behaviour, although the bike will still remain rideable even with quite drastic changes. Going shorter tends to make bikes squirrely, while going longer makes it a bit sluggish in steering. Going longer will also increase the strain on the head tube. Bikes will generally survive this, but it isn't advisable, particularly at your weight.

Some forks used to offer replaceable springs, for the ability to tune the fork to rider specifics. I still see that for rear shocks, but it's been a while since I saw it for forks.
Air-filled forks do require more upkeep, and are likely to break your budget, but do offer great adjustability.
Want it stiffer? Pump some more air into it.
Lighter forks tends to be flexier, and some even come with manufacturer's recommendations about rider weight. Allow the fork some weight, and do read the fine print.

The fork listed in the original spec is supposed to have a 1 1/8 steerer.