Noodle there are two ways of buying a used aircraft.
1/. Find an aircraft that the owner has spent lots of money upgrading.
2/. Find an aircraft with 'original' interior, paint, windows and a nearly time ex engine or one that is on condition and passed it's recommended TBO (perfectly legal under Schedule 5 mainenance in Australia) and then make it just the way you want it.
Realise that aeroplanes are like boats...easiest thing in the world to buy, hardest thing in the world to sell....in fact a lot harder to sell than boats.
Realise that the asking price is often what the owner wishes it was worth not what it is worth.
Realise any money you spend, or the current owner has spent, is not fully recoverable when you come to sell said aircraft...a new zero time engine probably is, as is a new zero time prop, but radios, interiors, windows and paint jobs etc add maybe 60% of their cost to the value of the aircraft.
Realise that when you buy a 30 yr old aircraft, at a small fraction of the new aircraft cost, you will still be paying new aircraft prices for many/most spare parts. At least you won't be paying for manufacturers liability insurance which is a HUGE % of the cost of a new aircaft.
Realise that if you go route 2/. above the labour costs, unless you can do it yourself, will mean that the final cost is in excess of going route 1/. above....but you do end up with an aeroplane you know intimately. The 60% rule of thumb kicks in when you come to calculating what the aircraft's new worth might be.
Realise that any quote you get from an engineer for money/time spent in rectification will need to be trippled to have any chance of being realistic...I am NOT joking!!! A good rule of thumb would be x 2.5 for the cost and x 3 for the time the aircraft is unavailable to use.
Proper hangarage is money well spent. It will vary slightly from place to place but I pay $200/mth for my aircraft at Redcliffe.
Insurance is easy to get quotes for...do NOT underinsure your aircraft. In fact if you feel that the aircraft is probably a little over insured you've probably got it about right. If you're a member of an Aeroclub that ownes aircraft and has a body of members who own aircraft you will probably find they have a group policy which is REALLY worth looking into. If not find a club that does and join. When you are trying to calculate how much to insure the aircraft for think about it crashing somewhere remote and not being written off in the crash....think about the costs involved in recovering and repairing it. Make sure the insurance is 'agreed value' imho.
Personallly I have gone route 2/. twice. The first time was an unqualified success and the second time I got bit by a nasty corrosion surprise that didn't show up in the pre purchase inspection....and a few yrs later had to replace the spar.
The pre purchase inspection is money VERY, VERY, VERY well spent...get someone you know and trust and spend whatever it takes to ensure you're not surprised after you buy...even if it costs $1000 to decide a certain aeroplane is not for you and then walk away from it that is money extremely well spent!!!! Even if you have to fly that person interstate and put him up overnight do so rather than getting someone you don't know locally.
Probably if this is your first aircraft you might go route 1/., particularly if you're not terribly experienced....you will probably end up selling it and getting something more suited to your needs rather than one that fullfills your ego/dreams....mind you I see that you're talking about a C172 or similar rather than a Bonanza so perhaps that is not an issue. Still one day you may want a C182 and that would be the type of aircraft that you may consider restoring and then owning long term to justify the expence of route 2/.
That's about all off the top of my head.
Chuck.