Many countries consider the "single engine airplane" rating to be valid on any single engine airplane which weighs less than 5,700 kg (or 12,500 lbs). The only practical variation for these countries, is the "seaplane" rating, with same weight limits.
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Now, in you want to fly a "Warbird" like a P-51D Mustang, you better get a type rating for that racehorse, which is over 5,700 kg. I would not try to risk my life in one, unless properly trained and qualified.
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In some countries, flying a "retractable gear" aircraft, or "constant speed propeller", or even "night" flying is considered as a "rating", as much as flying a "tailwheel" aircraft... some people love to make things difficult. Personally, I have rented airplanes I did not know, so I always asked for a chek-out in the specific aicraft type, at least to learn with a local pilot, about the area in which I was going to fly.
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As far as flying "IFR" - if the aircraft is so equipped, I rather file IFR... much easier to chat with ATC than following railroads, and get lost. A Jeppesen enroute chart is more adequate (and approach chart), easier to read than using a Michelin road map despite their 1:500,000 scale in France.
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And for me, IFR does not mean IMC necessarily. In a C-150, I would feel out of place in heavy rain, clouds and turbulence. I rather fly VMC and enjoy the scenery. I intend not to become a statistic at my old age. My fun flying is for sunny days and blue skies (low level along nude beaches, bikini patrol).
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I own a little Piper L-21B of Korean War vintage, with O-320 motor.
Never flew it IFR, nor is it equipped. Flew it a few times at night, when late to go home. Just has one old 360-channel VHF R/T (720 not needed), a Xponder (to keep ATC happy) to cross a control or terminal area, and a CB radio for R/T in banner operations or glider towing. I do not need a VOR receiver, nor a GPS... what's all that...
And plexiglas window/door open for air conditioning. Windy in the back...?
No mike/earphones in the back, just yell your insults.
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Happy contrails.