What Nick says is very good advice. The 26 was the first machine I ever owned back in 1961. (I had to get it and its companion table out of an attic in a house in San Francisco--something of a chore.)
As I dimly I recall, if the 26 has congealed lubricant, an important thing to look for is the possibility of a broken or damaged type pallet in the type wheel. If the pallet sticks (due to dried-out lubricant) in its slot in the type wheel assembly, it could get damaged by the type hammer bar as the typewheel rotates, which it will do when the machine is "run open". Check to make sure all type pallets are in their retracted position and move freely against the springs before powering up. Check that the type hammer bar is similarly not sticking.
The clutches in the 26 need to be checked for dried out felts. Squirt some 5W or 10W ordinary motor oil on them to rejuvenate the felts. (The detergent in the motor oil often acts to free up dried-out lubricants.)
If this 26 has been sitting around for a long time--its likely to be 65-70 years old--a thorough visual check out and manual rotation of the motor to be sure every critical thing is moving freely is highly recommended. Damage to anything when powering up the motor will be quite difficult to fix since parts for the 26 are virtually unobtainable in my opinion.
Good luck with the 26. I found it to be a very nice and relatively quiet teletypewriter and I made a lot of contacts with it. My late friend Bob Weitbrecht, W6NRM, a RTTY pioneer and the developer of teletypewriters for the deaf remarked to me that the 26 was his favorite teletypewriter.
Be sure to get the companion table and paper bar (fits on the back of the enclosure or table--I don't recall which) when getting the machine--especially the table. The 1957 edition of The RTTY Handbook by Wayne Green W2NSD and Byron Kretzman W2JTP has some info on the 26 since these machines were often put into ham hands from the local phone companies back then as they were replaced by 15s, 19s and, in some cases, 28s.
Have fun!
George W6BN