You can reasonably change fork travel, up or down, by 10mm. Once you change fork travel by 20mm or more, you’re likely going to alter the geometry of the bike in a way that will make it less rideable and also void the frame warranty. In addition, you’re not likely to notice much difference between a 120mm, 130mm, and 140mm fork. Honesty, a 120mm fork is enough travel for most Trail riders. Longer travel doesn’t necessarily mean better. If you get the Roscoe, I wouldn’t change it.
However, why not just buy the bike you want from the start? A new budget fork costs about $400. Why not just add the $400 to the price of the Roscoe and get a better bike. In general, 27.5 hardtails are mostly history. For myself, I would be looking for a 29er hardtail with 2.6 or wider tires. Check out the Salsa Timberjack/Rangefinder, Specialized Fuze, Trek Stache, Rocky Mountain Growler, Orbea Laufey, etc.. Any of these bikes will be much more capable than the Roscoe. You can get a Rocky Mountain Growler or Salsa Rangefinder with a 130mm fork and 29×2.6 tires for about $1000.
It just doesn’t make sense to buy a new bike and then make major expensive upgrades. Just buy the best bike you can afford, keep it stock, and ride the heck out of it the way it is.