I don't think splurging on accessories is the best strategy in this case. This is a small scope, the design cuts quite a few corners, there's not much room for improvement via accessories. It's a very aggressive F/4 that is hard on the eyepiece. Maybe save that money and get an 8" dob.
I know it's customary for most new users to just "get a barlow", but often that doesn't make a lot of sense.
Before you get any accessories, make sure the scope is very well collimated. That will make a large difference in performance. It's more important than any accessory. There are many collimation techniques, this is worth spending the time learning.
Anyway, let's talk accessories if you like.
The scope has 114mm of aperture, and an F/4 focal ratio, with parabolic primary 450mm focal length.
The short eyepiece
Theory says that a 2mm eyepiece will provide 225x magnification, or a 0.5m exit pupil - both are the maximum (and minimum) theoretical values for an ideal scope with these parameters. So either the 6mm with a 3x barlow, or a plain 2mm eyepiece (assuming you could afford one) would be more than enough.
OTOH, I have my doubts that this scope will do well at such a high magnification. It's the combination of small aperture, steep focal ratio (it's only F/4 which is very aggressive) and cheap optics that are not friendly to high mags - you'll get plenty of aberrations and a washed out image.
See if you can borrow a zoom eyepiece and a barlow, and explore the 3mm ... 8mm range. Two things to look for: A) Jupiter and Saturn looking washed out; and B) Stars looking like formless blobs, or bizarre shapes due to aberrations, instead of tight points (or regular figures of diffraction). As you're pushing the mags higher and higher, at some point the planets will look washed out - indeed, it will be harder to see Jupiter's stripes past a certain magnification, not easier.
My hunch is - you can't get much more than 100x ... 150x out of this scope. But it's hard to do an arm chair prediction as to how far can you push it.
The long eyepiece
It all boils down to the exit pupil. Young kids' eyes can dilate up to 7mm in full darkness. Old folks can only expect 5mm or so of pupil diameter. If the exit pupil of the scope exceeds the pupil diameter of the eye, you're wasting light. A 24mm eyepiece will provide a 6mm exit pupil; 20mm eyepiece - 5mm exit pupil; 28mm eyepiece - 7mm exit pupil. So there isn't much point in using an eyepiece longer than that; 25mm or nearby is okay, or anything in the 20s really.
But this assumes your eyes are dark-adapted when you observe. If there are sources of light nearby, your pupil will constrict a little.
You MUST use a solar filter if you want to look at the Sun. It MUST be the full aperture kind, the one that goes over the entry point in the tube (on top of the tube, where the light comes in). Do not use "solar filters" that go in the eyepiece!!! Those are not really solar filters, and are very dangerous. Permanent eye damage is very likely to occur when the "filter" breaks due to excessive heat and lets the full blast of solar energy hit your eye.
Make sure the filter is attached solid to the tube. You don't want the wind to blow the filter away, or a careless nudge to the scope to send the filter flying - you'll go blind in a second. There are filters that fit snugly to a given scope diameter; ask the vendor if not sure. You may even want to provide additional safety by taping it to the tube; better err on the side of caution.
In broad daylight, the big tube must be either covered with its vendor-provided cap, or the solar filter must be installed - at all times. Do not take the scope outside under sunlight with the top end uncovered, ever. If you own firearms and you know a little about trigger discipline, this is a bit similar.
With a good solar filter at the entry point of the tube, feel free to watch the Sun. There are additional hydrogen filters that go in the eyepiece (so those are optional, besides the main solar filter on top which is mandatory) but worry about that later.