By choice, I do this type of renovation work all the time but it’s very important you can spot & take care of whatever problems exist with the brick/block work base; blown plaster & substantial cracks are common; if you don’t repair it properly, they will just re-appear in the newly finished wall. Your customer needs to be aware of what problems they have, your recommendation of what you should do, how much it will cost (which is likely to be substantially more than a quick skim) & what the likely consequences are if you don’t; it’s then their decision. If they ask you to go ahead regardless, the decision is yours but there is always a risk that you will still get blamed for a crap result with consequential damage to your reputation; personally if they don’t want to go with what I recommend, I find I can’t fit the job in.
I would not go near patching or one coat plaster; I tend to use sand/cement (sometimes with lime) render or Bonding for base coat repairs, depending on what’s there already & local circumstances; but, sometimes, (wash my mouth) dot/dab/skim on new boards can be the most effective solution. Getting render to stick can be difficult if you’re inexperienced & Bonding plaster needs good prep & care on high suction backgrounds; I tend to save Hardwall for new block walls. When you come to skim over patch work backgrounds, they will give varying absorbtion rates meaning the skim won’t all go off at the same rate which you will need to be aware & take account of to avoid lumps & rippling.
As PDB said, if you want to progress, you have to start somewhere but I would advise you do some serious boning up first & the forum archive is as good a place as any, then it’s your call. Personally I love this sort of work & much prefer it to skimming boards which I find a bit boring now.