Personally I would go for the 5hp. I use a 3.5 Mercury on my tender which is light and with me and my son it works pretty well. But I still find I want the 5hp. Ok, so you are using yours as a back up to get you to land ASAP incase the main engine fails, and not as a full time engine. At slack water or with the tide the 3.5 will do. But against the tide you would probably struggle or it would be a real slow journey.
At £160 though I’d be tempted to buy the 3.5 and try it out. If it’s no good sell it on and buy a 5hp.
you won’t loose any money.