Hello neighbors!

I have a question for all of you pros :)

This year, I've decided to ask a neighbor who has been looking over the garden around my NYC apartment for 15 years if he would like some help this year--and to my surprise he bequeathed the garden to me!

So here I am now, with a newly inherited garden! I am not new to horticulture and I like to think I know a thing or two from my lifelong passion of plants, however I've mostly had experience with indoor/patio container gardening--never a "real live garden" in the ground...haha

The space isn't terribly large, it mostly has North, East and West exposures, with some areas being openly exposed to a great deal of sun, and others sit under the protection of shadow from the tall high rise building itself and very tall mature trees. So all in all, the different exposures give me a great deal of flexibility.

Currently, there isn't much going on there. Mostly old bulbs that have probably been there for the entire 15 years... The typical spring bulbs, like daffodils etc. Then there are hostas and of course the ever present Day Lilly colony ... Haha. I plan on removing the hostas and putting them under trees, as for the Day Lilly, they just may have to go... They're that typical orange that can be seen on a highway.

So here is my question....

I don't mind spending the money to do it right, but I also don't want to get over the top crazy since I do travel quite often for work. I am sure that the soil hasn't has any amendments/fertilizer applied to it ever. I plan on spending this weekend preparing the ground..

Are there any "all purpose", highly recommended soil amendments/fertilizers that you suggest I add to the soil to prepare it? Anything specifically that will make a difference in the success of my garden and help it thrive?

Some things I possibly plan on putting in may include :

-hydrangeas (I don't mind the color difference as I know nitrogen affects this)
-roses
-various annuals and perennials of course
-alocasia/calocasia
-peony
-gladiola

Etc....

Any information is highly appreciated and thank you for taking the time to read this!