When people pay in excess of $300 for something... let alone +$1000, they expect the thing to last for years without issues. They only see the bottom line price... and do not figure in labor, delivery, setup...and future problems. They do not want to pay a heavy price.. so better components are often not opted.
The 'Brain' (Motherboard & CPU) rarely goes bad. Maybe in 8 to 15 yrs, a Cap will bubble up.
Power Supplies, possible failure.. but easy as pie to replace.
A CPU fan will probably get clogged up and stop.. causing Slow performance and possible CPU damages. Recommend a periodic service checkup and or show them how to do it.
The biggest problem is the Hard Drive, for corruption, data changes, and failure.
Its also the most work to deal with.
Take an example from our controlling elite.. and use Fear, to get people to chump up the extra $150 for a backup HD with finalized disc Image. Just start saying.. well, you know the thing could literally die in 2hrs from now.. right? And that it would take me a good +3 weeks to be able to getting it back up and running?
If they do not want to put in another HD, at very minimum.. pop a flashdrive image of the OS & Mame install + config files. They are dirt cheap these days. Tape it somewhere in the cab, where it wont be found.. until they call...
As for the Backup HD:
It stays In the system, but the power cable is disconnected.
Label Both HD power cables with different colored & different named stickers. Should there be a problem... tell them to open the case, disconnect the "Groovy Green" connector... and plug in the "Code Red" connector.
Once the Code-Red is initiated.. get the Fear mongering out again.. and charge for a new HD replacement + reimage. (collect money in advance)
Now, Ive not done this.. But working on enough PCs, Ive played the game long enough to understand the consequences of having to reload, hunt obsolete drivers, deal with bad driver quirks, windows updates, reinstallation of customized software. Slow-as-Paint-drying data transfer speeds to old pc hardware... and other such related issues.
I also recommend a 2nd backup image somewhere in Your house. At minimum.. copy of all their needed drivers, windows key, software keys, hardware listing, software list.
EDIT:
If this is for friends / family that you know you cant refuse... just put the 2nd HD in there anyways, and bump up the costs of the rest of the materials. And or take the hit on the labor cost... because eventually, your going to get the call. Better to be fine with that call (easy fix -vs- entire weekend gone).. or should I say... many calls... than the minimum profit that your gona lose out on anyways.