Newer doors have much better weatherstripping systems than do older door frames. And bringing older frames up to par is sometimes not simple, and a new door blank has to be carefully fitted. I vote for the complete new door, even if the cost might be higher. Check local lumberyards for the best cost, avoid big box.
Overall I do agree with Bill that new complete door units are better than the older units in a number of ways. Buying just a "slab" and trying to fit it into your existing door frame will not be easy for the novice DIY. The new 4 1/2" wide jamb can be fitted into the existing 6" wide rough framed area, and will look good if you take your time and do it right. I don't try to make jamb extensions match perfectly, actually I set them back about 1/8" for a little "reveal". It looks better than two pieces of wood you are trying to mate and don't fit properly. Good Luck, David