Recent events have made me decide that when the time comes that I get serious about buying a plane (next year), I will include in my budget 5K, or more, for problems that are sure to come up in the first 100 hours or so... and even with that I could still be grounded by a gremlin. It seems, more times than not, owners sell their plane because there is something lurking/looming that they don't want to pay for, so decide to unload it. Not saying that's a bad thing, necessarily, but dam, it sucks for the buyer and is an unfortunate reality that can put the kibosh on the elation a new owner would be feeling...
It seems there are things that just don't show up, even in the most thorough of Pre-purchase inspections. Scary!
Disclaimer: Long post. I started off thinking this would be a 1-2 liner...not the case. Sorry
I think 5K may be a little light depending on the type plane you plan on buying. Also depending on how much excess money you have left each month from however you get your money. In addition to the type plane, recent usage, type previous owner (meticulous maintainer vs a put-it-offer), etc all come into play. If one had to put a number on it, I think the best realistic would be ~10% of the purchase price. Plus as
@eman1200 says, you may want a couple upgrades?
In my first 7 months of owning I've done the following maintenance:
-HSI repair (G5 HSI wasn't available back then unfortunately)
-Double prop O/H (obviously a single driver would catch a 50% break here)
-Left fuel divider O/H
-Left Alt belt replaced
-G/S connector cleaned/replace
-Mandatory heater inspection via brand new AD
Lucky for me, no catastrophic expenses. Keep in mind a few things. This all totaled more than 10% of purchase price (had I been a single I would be right at the 10%). Also, my first year is not up and my first Annual Inspection will be at the 1yr point. PS. This doesn't account for a new heater as the existing one failed the brand new AD, lucky timing. I didn't include oil change/labs. I think I've put about 50-60 hours on her and about 16 states under her. Am starting to feel pretty good about her health now (knock on wood).
For upgrades, that is obviously mostly self inflicted. I wanted my bird to be as safe an IFR traveling machine as possible. As such, I chose to do the following first year upgrades:
-Swap the GTX327 for a GTX345
-Install an Insight G4 Twin Engine monitoring system
-WAAS upgrade my GNS530
-Install a Garmin G5 in the primary AI spot and shuffle that to the T&B spot and move that down one left one.
This has given me traffic and wx in the plane, 12 cylinder monitoring to include, OP/OT/MP/RPM/Fuel Flow/Fuel totalization/ETC, LPV/GS approaches everywhere (almost), and an independent 4 hour battery backup AHRS AI/ASI/ALT/HDG/ETC. Basically the plane had been upgraded from the 70's to the 90's when I bought it and I took it up to 2020 minus a modern A/P.
Ramble over....having a bit of a maintenance/upgrade budget is very prudent on your part.