You may have short sleeves, approximately 6" long by 12" high, cut with the body panels, but sleeveless tunics may have been more common.
Round necklines were known, but apparently the usual neckhole was a slit about 20" long, made simply by leaving most of the shoulder seam unsewn. For heavy work the right arm can be slipped through this neck slit. To close up the neck slit so the tunic will stay on your shoulders, gather a "knot" of slack fabric at the back of the neck and tie a cord or thong around it, or just pin the slit shut with a couple of fibulae.
The tunic hangs to the knees or below, but is normally worn bloused over a cord or tied belt to raise the hem above the knee.
Practically any available white or off-white is acceptable; it need not be blanket-weight, but should certainly be 100% wool.
Undertunics cannot be well documented, but the wearing of one for comfort is an option. Make it of white or natural linen, the same shape as your wool tunic. In very hot weather a linen tunic may be worn instead of wool to avoid dangerous overheating.
The issue of tunic color is hotly debated, to say the least. The use of white is based on evidence summarized in Nick Fuentes' article, "The Roman Military Tunic", in Roman military Equipment: The Accoutrments of War (Proceedings of the Third Roman Military Equipment Research Seminar), edited by M. Dawson. In short, at least two Roman illustrations from the early principate show armored soldiers in white tunics. One of those, from Pompeii, also shows one soldier in red, possibly an officer or centurion. A papyrus from 138 AD deals with the purchase of white tunics for soldiers. Granted, this is not much to go on, but it is evidence, as opposed to "logical" or "practical" arguments. The interpretation at this time, therefore, is that legionaries wore white (undyed) tunics and that centurions wore red. But we acknowledge that we could be wrong and are eager for new research to add to the debate.