Having been an speleologist until I grew too old and fat, I can't understand what could the bats do that drives you nuts, unless they poop all over you? I'm very interested in what the problem could be, really. Would you mind to explain a little more about it?
First logical step would be to identify the species. This would give you additional clues, and perhaps provide a solution that would work better without needing neither to learn electronics nor using ultrasonic booms. Moreover, are you sure that method would be effective? I somewhat doubt it.
So, first advice would be to contact some brazilian biologist working with bats. Just google for papers published. That alone would probably get you a trove of useful knowledge. If you are just interested in getting rid of bats, and electronics is just a means to achieve that, even to pay some hundred equivalent euros to a biologist would probably be cheaper, faster and surely more effective than getting into this rabbit hole
OTOH, if they are insectivore, as most bats are, it could be wise to leave them do its thing. Otherwise you could escape from the pan just to fall in the fire. I for one would rather prefer having hundreds or even thousands of insectivore bats feeding around my home than to have to deal with bloody sucking insects, even if mosquitos are just a small part from their diet.
Now on the oscilloscope part: If you aren't particularly interested in electronics, and all you want is to get rid of bats, all you need is a means to check if you are producing the frequency you need or little more. As radiolistener said, it could be that the sound card in your PC would be enough. Alternatively, a DMM with frequency meter could do too. Or a Zeeweii DSO154Pro for about €40. I own one and I can assure you it will work up to >10 MHz.
https://br.aliexpress.com/item/1005004972406356.html . Link to the relevant thread:
https://www.eevblog.com/forum/testgear/new-toy()-scope-dso154pro-1ch-claimed-40mss/msg4531274/#msg4531274 It also has a frequency generator that will surely work in your range. Again, if all you want is to get ride of bats, I think you need not more than that.
If you are interested in electronics and are on a really low budget, I think the Zeeweii DSO2512 suggested by Aldo22 would be better than the DSO154Pro since it's dual channel and has enough bandwidth to look into PSUs, etc. I don't have one, so I can't really say how good it is. But after reading the thread
https://www.eevblog.com/forum/testgear/new-2ch-pocket-dsosg-sigpeak-dso2512g/msg3547154/#msg3547154 and my own experience with the DSO154, I would say go for it. It will be enough to increase your knowledge by a couple orders of magnitude. Then you could decide by yourself what the next oscilloscope will be, and it will remain useful as a litlle, battery-powered, portable oscilloscope you are not so afraid of wrecking it, and still able to do what it can do.
The second-hand option? I wouldn't recommend it. Of course it could be that you'll be able to find a bargain, but I doubt you'll easily find anything for less than 150 euro/dollars, plus shipping (boat anchor > 7 Kg), it will do just 20MHz, perhaps 50 MHz if you are lucky, it will probably be 30 years old, and even if it's working today, you don't know it some component or even the screen will die just tomorrow. Then you could be in for a rough ride to, first, find the spare parts, then to fix it. After your posts, I don't think it would be an easy task for you right now, and is often said that to fix an oscilloscope you need another oscilloscope.
If you are interested in electronics and have a budget some hundreds of euros/$, then we have the usual suspects: Entry-level 4-channel benchtop oscilloscopes: Rigol DS1054Z, GW-Instek GDS1054B and Siglent SDS1104X-E. I own the GW-Instek and am happy with it. The Siglent is the one that has more bandwidth, 100 MHz that can be upgraded to 200 MHz, while the other two are 50 MHx devices that can be upgraded to 100 MHz. So, the Siglent could be the more future-proof option in you electronics career. Just don't think its MSO option will work "fine". All these three oscilloscopes are good entry-level devices, that's why I decided to buy the cheaper GW Instek instead. You'll have to search the forum for the relevant threads and decide by yourself.
Last, if you have about 1000 bucks or more to spend, look for Rigol MSO5074 or Siglent SDS2104X Plus. This Siglent is all any hobbyst will ever need, I think. I was on the brink of buying it. But finally I decided to go with an entry-level one, which made me able to buy other equipement like soldering station, desoldering gun, hot air rework station, linear PSU, AWG, logic analyzer, etc, that allow me to play more than just an oscilloscope would.
Boa sorte