It's a little odd for a tiny private tech startup to be talking about monthly stock purchases rather than a larger upfront grant (of options or stock) that vests over time. The latter seems much more common.
You owe tax on income when it is received -- whether that income is cash or stock. So in your 'for instance', you'd owe on $5K each month. It's as if they paid you $5K cash, on which you owe taxes, and then you decided to buy $2K of stock.
In many cases, you would rather pay for your full grant at the earliest possible date, before your own sweat equity starts making the stock more expensive. The company protects itself by having the stock subject to vesting if you leave early -- but then you should make absolutely sure to do a prompt "83(b) election" so that you don't owe taxes at each vesting increment.
A good startup lawyer could be telling the founder and you all this, and more. If the plan is for this to grow big, and there will be other employees on stock plans, doing it right is worth the cost.
You owe tax on income when it is received -- whether that income is cash or stock. So in your 'for instance', you'd owe on $5K each month. It's as if they paid you $5K cash, on which you owe taxes, and then you decided to buy $2K of stock.
In many cases, you would rather pay for your full grant at the earliest possible date, before your own sweat equity starts making the stock more expensive. The company protects itself by having the stock subject to vesting if you leave early -- but then you should make absolutely sure to do a prompt "83(b) election" so that you don't owe taxes at each vesting increment.
A good startup lawyer could be telling the founder and you all this, and more. If the plan is for this to grow big, and there will be other employees on stock plans, doing it right is worth the cost.