I replace manikins when they can no longer be repaired. Sil-Poxy from SmoothOn can fix a lot of problems. Torn-up airways can be repaired if you're careful and patient. There's also a lot of tips for removing stains from manikins. The most popular fix is benzoyl peroxide and sunlight.
The number of sticks you get per arm depends on which arm and skin you have. Don't let your participants use anything bigger than a 20g. 18s and larger just needlessly damage the manikin. Most latex veins will self-seal after sticks, so no need to replace them until they just are inoperable. Also, rather than spending all the money for a Laerdal vein set, use surgical tubing and barb connectors. If the skin is shot, you'll just need to order a new skin set from your vendor. No need to replace the entire arm though. IV arm skin is usually once a year for me.