Many options to consider.
Consider a Retainer plus work performed type of contract. You get a solid amount of money to sign the contract (retainer). This binds you to being available to support emergency work and problems. Then, have an hourly rate. Might also consider having a night and weekend/holiday rate. Could also base the retainer on the number of terminals and servers supported, so if they add terminals/servers, you get more money (due to added responsibility).
Or you might gamble on a fixed bid contract. For, say, $30,000 per year I will take care of any upgrades and repairs required on your systems. Replacement parts and upgrade equipment to be paid for by client. Or, say, fof $500 per terminal and $10,000 per server, I'll provide annual support.
Or you could opt to go with hourly only. For this type contract, I would definitely have higher rates for night and weekend/holiday work, but reserve the right to perform scheduled work and services during that timeframe at your option at the regular rate.
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The trouble with doing something right the first time is that noboby appreciates how difficult it was.
- Steven Wright