Hope this helps...
Dave is right, a small branch line would not generally have its own roundhouse and turntable. However, it's possible that there might be a roundhouse if the branch line used to be an independent railroad (typical of the way Southern Ry. grew over the years). For example, there was a roundhouse for Southern in Danville, Virginia, and a complete separate locomotive shop (not round, but along the same general size) less than a mile away just across the river, on the Danville & Western - which for all practical purposes was a small branch line on Southern Ry.
To your question: Yes, you can figure that if there was a roundhouse, there would have been some arrangement made for coal & water. However, you do not have to have a coaling tower. The D&W never had one - they had a raised platform onto which a gon or drop bottom hopper could be spotted, and coal was then shoveled from there into the tender of a locomotive on an adjacent track. Southern used a similar arrangement in Balsam, NC, though the track was a little taller and I believe there was an actual chute for dumping coal into a tender, much like a mine tipple. Here's a photo of another at Leaksville Jct., VA, on the D&W:
You can also go really simple, and use a conveyer with a coal car spotted at ground level. In HO, Walthers makes a suitable conveyor, I think. Another, even lower-tech option (more suited to the smaller locomotives of the 19th and very early 20th centuries) was to simply shovel coal out of a gondola into a tender on an adjacent track, with both tracks at the same elevation.
Water was usually stored in an elevated tank, unless the topography allowed a tank to be built at ground level on a nearby hillside. In many cases, the tank had a spout to fill locomotive tenders directly, but in many others the tank was located in some convenient spot and a free standing water spout was located in the yard. For example, in Danville, Southern did not have room to build a water tower next to the main line (for watering trains stopping at the passenger station), so they built one on the other side of a narrow valley a few hundred yards away and then ran large iron pipes from the tower, across the valley, and into a water stand which was between the main line tracks.
You might also want an ash pit, where the ash pans could be dumped. While some railroads just dumped the ash pan directly onto the ground, it was a much better practice to have a masonry pit which could be shoveled out and which would not burn up like wooden ties would. Also remember that sand was typically provided very close to or in conjunction with the coaling facility.
The simplest arrangement would be to have all 4 (coal, sand, water, and ash) in a line on a single track which also served as the roundhouse lead. Busier shops would typically have one or more ready tracks which would allow a locomotive to come out of the roundhouse and be waiting on an engineer or hostler, while other locomotives were being serviced. Also, in many cases provisions were made to service more than engine at a time on adjacent tracks.
In your case, I would suggest either putting all of the servicing on the roundhouse lead, or on a separate track close to the backdrop. Ideally, have both tracks connected to the turntable so that a locomotive can come in or out of the roundhouse while another is being serviced.