Not sure what you mean by blocks all over the deck, but they are usually replaced (if we are talking about turning blocks, or deck organizers on the cabin top) en masse. Like these...
As for backstay consider an adjustable cascading backstay, it's easy to make, and honestly you can make it with amsteel single braid line if you want.
As for the mainsheet, the catalina 25 mainsheet tackle will work, however, if you have to buy new, the least expensive option that is still decent equipment is garhauer. However, you might get your best deal actually from catalinadirect (I know it's not a catalina)... but they have some great small boat deals on hardware for fully replacing stuff like that. This is a good example...
Catalina Direct: Mainsheet Block Kit, Stainless Steel, CP-18, CP-22, C-22, C-25
Oh and as a simple example of a cascaded backstay, this image pretty much tells it all...
Finally a how to (which is for my boat, the S2 7.9 so you will need to adjust the length of line that acts as the backstay to what is appropriate for you) to build a flicker (overkill for the Helms 24 and not necessary), but also the cascaded backstay... Note there are several tabs to the spreadsheet. Spectra and or amsteel blue will work, its easy to build the splices.
The last thing you mentioned was outboard, and 4hp long shaft... YES it'll work fine on flat water, with very little chop... if the winds get over 20mph or so, or waves over 2' you might be struggling a bit into the wind motoring. Given that I am assuming your main sailing venue is a lake, I suspect you'll be fine.
By the way, welcome to the site, and welcome (back) to the insanity (sailing).