Depends what size tinnie? If you're spending 5k you can pick up a decent 4m tinnie witha 25-30HP if you look around.

 I agree with sea-kem. Search around and you'll find something. I don't reckon 15hp is enough unless you're planning to be strictly river based, but it wouldn't be too long before you'd want to upgrade again. Do yourself a favour and get the best you can straight up.

 Bigger is better IMO. 5k should pick u up something like sea-kem said. Dosnt take much to weigh a boat down out there extra power is helpfull

i have a 4m tinny with a 25hp merc,

good size motor to boat and planes along nice

and i thought that was a good match up I paid  $4000 and it had extras ie. flat floor, sounder, bilge, nav lights, grab rails, all safety gear good trailer etc. If you keep looking in gumtree for a while units like this come up for sale regularly motor was a 2004 and never let me down once. Even with me mate who's a big fella it got along great and cheap as chips on fuel so I guess I wouldnt go less than 25hp (even towed two kids on ski biscuits great).  Having said that I suspect it wont be long before you and ya mate want something bigger (us boys always want bigger toys!) . I used to take mine out past 3mile on calm days but with a bit of wind its horrible and ya start thinking why didnt i get something a little bigger - hope you find what your looking for

Agree with the above responses, I have a 4.65 mtr open fibreglass dinghy with

a 25hp Tohatsu, the boat is great and I can use it in sheltered waters and also have taken it 7km offshore

and caught good fish when conditions allow. Cheap to run and great ride being fibreglass.

For $5000 you should get a half decent tub especially coming into winter being a buyers market , bide your time . 25hp would be the min for even inshore boating cockburn sound , river etc . 

15 horses works fine. even with two adults on board.
Once you get three people it struggles though.
I had a 12 ft quintrex dart and it never needed more hp.

I bought an older hull and trailer with a 2006 merc 15 hp for $3200 put a deck in with a few extra rod holders and sold it for $3800. You will easily get a great dinghy for 5k especially now were nearing winter. I frequently went In the ocean with it no dramas. Save a few pennies and put some mods in, use it for a while and sell it for more than you bought it. Can't go wrong with one

I went to gumtree boats and jetski section, selected motorboats and powerboats in refine search and banged in a limit of  $5000. Theres a  4.9m Derben runabout with a 2002  4 stroke mariner there for  $4000 (The motors prob worth that on its own wish i had a 4 stroke) The boat looks like it might need the extra  $1000 you would have left to do it up which is great fun for you and ya mate to doi together over winter then next summer youll be out trolling for mackies at west end mate. the add said something abiout the boat has made regular trips to rotto in the past ya just pick a good day to do it we even check weather before we go out in bigger boats Anyway its winter now so more bargains should come up and hope to see ya out there next summer 

I went to gumtree boats and jetski section, selected motorboats and powerboats in refine search and banged in a limit of  $5000. Theres a  4.9m Derben runabout with a 2002  4 stroke mariner there for  $4000 (The motors prob worth that on its own wish i had a 4 stroke) The boat looks like it might need the extra  $1000 you would have left to do it up which is great fun for you and ya mate to doi together over winter then next summer youll be out trolling for mackies at west end mate. the add said something abiout the boat has made regular trips to rotto in the past ya just pick a good day to do it we even check weather before we go out in bigger boats Anyway its winter now so more bargains should come up and hope to see ya out there next summer 

pm sent

my first boat/ dinghy was an old ali clark with a 8 hp seagull johono on the back when I was 12 and I went every where in it was a great way to learn boating the 15hp these days are good units so don't worry

that there is a fine line between (knowing your capabilities + fishing well within them) + (being a knob + creating a dangerous situation) then you will be just fine.

although I wish it was a 25. haha

i have a 3.7m stacer tinny for sale with 15hp mariner very low hours. $4000 if you are interested pm me