The $ killer with airplanes is the AWD's (airworthiness directives) which are mandatory.
On my C182 the prop needed some seals changed every 5 years and the prop shop wanted $500 as they would only do a complete OH.
Then came an AWD for the fuel cells at, I recall $4-500 each. and so on.
In Canada there was a provision that if an automotive part that was common to aircraft it could be used providing it came in a factory sealed box. (might not still exist)
Now I was fortunate as I also held an A & P ticket compliments of the Cdn Air Force and a high school summer training program* (as well as 'hands on experience' as I did all my maintenance under A & P supervision)
I also documented all that time.
In Canada you needed a certified school training + supervised practice which meant 3-4 years (2000 hrs) academic + 3 yrs apprenticeship. Under Can/USA bilateral agreements I was accredited also my USA A & P ticket plus a USA pilot ticket.
That gave me great privileges, I could buy a US aircraft that was out of licence, certify it for ferry flights fly it with my USA ticket, import to Canada and clear Customs then inspect and repair as necessary and re certify and register it in Canada.
Those were great old days! (I did it more than a few times)
My 'hobby' led to an avionics career and a certified Avionics facility and finally a Cessna dealership as well as as an FBO operation.
I was also a director of the AEA (Aircraft Electronics Assoc) for Canada for many years.
The most 'fun' jobs we did was to modify SEL's for transatlantic ferry flights, and that fully certified with DOT.
I once equipped a C206 for an 'all around the world' adventure, another was 5 crop sprayers upgraded to full IFR c/w Loran, HF and long range.
That was easy as sprayers have a nice huge tank up front so I simply sealed, drilled and added a fuel line and pump to transfer the fuel. For the back up pump I used boating bilge pumps. I modified those 5 AC in a 3 week time frame! and all DOT approved and inspected!
For some ferry tanks we simply strapped down 45 gal drums inside the AC, others were more complicated and required custom tanks.
It was not all that difficult as it mainly consisted of secure attachment and generally we'd use the seat rails and cabling + turnbuckles. After all the seat rails were designed for 170 lb passengers and that at 9 G's. so a rear seat tanking could accept 340 lbs of fuel. Worst was plumbing for dual pumps to refill high wing AC.
Naturally we'd install GPS (or Loran-C back then) .
HF SSB was another requirement for constant communications. Naturally the AC had to be IFR certified.
* the DOT accepted the RCAF course in lieu of the 2000 academic hours.
And do I miss flying, YES!
Your ferry modifications story, brings to mind an old transatlantic ferry story.
WAY back in the summer of '64 I was based in Argentia, Nfld. as a pilot in VW-13 (EC-121), a USN squadron.
We were the over water extension of the DEW line, and although based in Nfld. flew our 12 hour missions, for 2 weeks at a time, from Keflavik, Iceland.
We were positioning from Argentia to Keflavik on the 19th of Sept.(my mothers B-day), and asked to be alert for the whereabouts of 3 Mooney Mark 20s who were inbound to Reykjavick, and overdue.
We made many calls in the blind on 121.5, but never received a response.
On landing in Keflavick, we got the full story.
The 3 Mooneys had departed from St. Pierre/Miquelon enroute to Reykjavick.
The leader was apparently a retired USAF navigator. The other two had no long distance navigation capabilities. They were to follow the leader.
The ceiling was about 300', and there were multiple layers above.
The freezing level was at about 1000'.
Somewhere along the way they got split up.
The leader (who could navigate) landed in Reykjaivk.
One of the other two flew past Iceland, but a West bound PanAm flight convinced him of that, and he attempted to return to Reykjavik. He did not quite make it.
He ran out of fuel about 60 miles off shore, and the USN helo from Keflavick picked him up.
The 3rd guy is still out there somewhere.
The 3 Mooneys were originally bound for mainland Europe.
ONE...Actually made it!