Yes, having a guide is mandated by the Gunung Leuser National Park. You'll be glad for one too as the jungle doesn't have marked trails and it will also be difficult to spot an Orangutan hiding in the trees without a guide!

The trails starting in Ketambe can be categorized into easy, medium and strenous, and you can choose which one you take for the trek. For the easy one, the terrain is mostly flat so minimal fitness is needed and you only need to be comfortable with walking for several hours at a time. For the strenous one, there are sections with steep inclines so you should be used to moderate exercise on a regular basis. Medium is somewhere in the middle. The heat and humidity makes the trek feel more intense than it otherwise would be, but you can take breaks as often as you need!

It is possible to go year round. Note though that if you plan to go during November to March, this is wet season in Northern Sumatra (with December and January seeing the heaviest rainfall). This makes the trails more muddy and difficult to trek through, so you'll need to pack good rain gear and a change of socks!

The trek only takes 2 or 3 days (depending on which option you pick), but the total trip (including transport time from Medan) will take 4 or 5 days.

Between the providers, the main difference will be the accommodation that you will stay in - they're all broadly equivalent in terms of amenities, though One Thousand Hills has the newest guesthouse.

Between the options, the main difference is that the 5 day option allows you an extra day's trek to see a bit more of the jungle (e.g. hot springs and waterfalls) and a higher chance to spot the orangutans.

Ketambe is a pretty small town with not very much outside of the guesthouses and a few warungs. You may want to give a try the kitchens of some of the other guesthouses if you are bored of the one you are staying in - for e.g. One Thousand Hills Guesthouse has a pretty good selection of food choices.

If you have an extra day that you would like to spend in Ketambe, you can experience white water rafting on the Grade III-IV rapids of Alas River.