The house is in pretty nice shape, but the lakefront is mucky and  full of weeds and saplings. 

The small sapling shouldn't be a problem.

I just spent all weekend with a rented tracked skidsteer loader with a monster bushhog cutting this bamboo down and grinding it up . it was more than my kubota's bushhog could handle , I still have at least another day with a york rake to clean up and pile the stuff up .At least I can pile it in the back of the lot , it would fill about six 40 yd cans .The fun part was this skidsteer and bushhog ripped thru the bamboo and it would grind up 4 inch saplings , I knocked over a couple trees roots and all came up .

A bushhog will do a rough cut , but they can cut tall grass vines and saplings up to 2 to 3 inches depending on the size of the machine . 

They will stand on hind legs and chew small limbs off clearing understory and they spread their front hooves walk up small trees or saplings and eat all leaves and branchesAll they need is a water source ,I have used them several times where it  was fenced goat owners may be looking for forage for them.

Is a mature tree or a sapling more likely to show a high growth rate?

I don't want to grow into a sapling, I want to grow into a mature oak tree.

. - If you planted trees each year for 10 years before you started harvesting, you could eventually use the money that you sold the trees for to buy more saplings and keep planting.

Anything worthwhile like being financially free does not happen in 7 months, in the same way that a sapling does not become a tree overnight.What you need to focus on right now is to generate CASH to live off on, not PASSIVE INCOME.To generate cash, you have several options:You can wholesale a deal - little to no risk; you can make money in 1-3 monthsYou can fix n flip - very high risk because you won't have income in 3-9 monthsYou can become a licensed real estate agent - no risk (other than the money you invest to get your licensed) & you can use this to find deals and get a listing to make some cash in 1-6 monthsGet a JOB in real estate (temporarily and at the same time learn the business) - no risk, no investment required; like being a leasing agent, or property manager, or acquisition associate for a flipping company, etcOnce you are generating cash so that you have $2500 coming in, then now you can focus on acquiring rental properties.Hope this helps.