The 27, Sphynx decals, is a long bobbin machine, and takes modern class 15 needles. Just like the 115, if you treadle the handwheel the wrong way, you will get thread jams.
The 66, round class 66 drop in bobbin, class 15 needle.
Manuals are available for both, free, online, at ismacs.com. The manual has a clear diagram of oiling and lubrication points (lube=suitable grease), as well as how to thread, etc.
Other than a missing slide plate, not much can go wrong with the old machines, and repros are available. Leather belts are available . You can also use screen spline, O2 tubing, aquarium tubing. You'll need 72", and a clip/staple yo join the ends. Expect to cut a wee bit off, then test. You'll want the tension on the belt as loose as possible to make treadling easy,. But not so loose the belt slips.
Both machines remove/install the same way. "Throw off" the belt (odd doo hickey at the right near the drive wheel), tip back the head. Locate the hinge pins. Now, on the casting, there is a set, or grub screw, that secures the head to the pin. Loosen the screws, slide off the head.
Dismantling the legs is something I've not done before. If you rent a car, get an SUV, and bring along moving blankets to cushion it. Wrap a towel around the head and secure in seat belt it in the back seat. You do NOT want a 30# projectile flying towards your head because you had to slam on the brakes.
As far as "too much varnish" I'd guess not. Perhaps freshly polished, or fed. Howard's Feed n Wax is a great product. Usually, issues to the wood are due to moisture, being used as a plant stand, or stored in an unheated building.
Pick one of the machines based on the prettiest drive, or great place to stop for lunch. Either machine is a winner. Do let us know!