With "very limited mechanical skills" there's really no budget "plug n'play" solution - it's not so much 2 vs 4 stroke but just plain economics... if you buy the least expensive engines, you're faced with either unreliability (stock/unmodified) or some serious fabrication demands... sometimes both lol
The cheap in-frame two stroke kits are w/o a doubt the least expensive way to motorize a bicycle and they can be made reliable - but there's a rather steep learning curve if you're not familiar with even such a basic engine design. They are very simple motors however and a great learning tool... the catch is due to the lack of quality control, particularly when combined with that "learning curve" you may find they're not as economical as they initially seem... fortunately, parts & even just the engine itself are very
inexpensive to replace.
The least expensive four stroke engines seem very attractive due to their low initial
cost and reputation for reliability - the catch with those however is the motor is somewhat more complex and they're not kits. So you'll need to purchase or fabricate everything else needed to motorize a bicycle & that can add up surprisingly fast... and if you do find you need to repair or even tune the motor, well... now you're dealing with valves and all sorts of fun stuff lol
Thing is - despite the popularity of the above options, there are other choices. If you're not interested in maximum preformance, fabrication and/or repair and only want inexpensive & reliable transportation - don't fall for the "false economy" of the least expensive engines as they'll end up costing you more than you'd think, even if just time. By spending a little extra up front you can side step all of this - check out any of the "rack mount" kits, both 2 and 4 stroke as generally speaking they've better quality engines than the in-frame kits, but don't require the fabrication of the engine-only 4 stroke options...
They are pretty much the only true "plug n'play/one size fits all" kits available ('cept electrics ofc, but those get pricey fast) & while they have some disadvantages (weight & balance, cargo room), for the money and your needs, they might be the most straightforward solution.
Jus' a thought