I know couple of businesses which could be of your interest to you.
At the same time, since I have been 3-4 businesses simultaneously for quite sometime and also share similar vision like yours, Would you be interested in a short discussion?
Fyi, I am also on the lookout of partners who can support hand in hand.
Lets discuss...

It really depends on two Main things, the first being what is the online business, and the second is what is the attitude of the owner.

You want to invest less than 100k per investment, and want the original owner to continue the work.

The online world is a completely different one to the brick and mortar one. Experience in running online businesses is something that only comes with endless hours trying to build an online brand.

My recommendation would be find the Niches or Products that you really believe in. Check out their website and their company history. It only takes a little bit of research to find out who is behind an online business. Then its as simple as making an offer for a share of the business revenue.

Another option would be to look for automated online businesses, there will be a lot of them, that make a regular income through a product or service. Calculate the return you want and purchase one of them.

However, if you want a partner that will be working on the business you need to find someone that is really passionate about what they are doing. Only those people have the motivation to keep going when things are going against them.

Depending on what stage of business you're interested in and how you see your own role in the project, I might be a potential partner for you.

And if not, I might be able to identify relevant prospects or add value to the enterprise (depending on what it turns out to be).

You don't need 100% of the skill set, provided you can ask the right questions, proceed slowly, find reliable partners with relevant talents or include them as part of team-building.

Buying an existing website outright would be the wrong course of action unless you are retaining the webmaster as a partner or employee ... or unless you're bringing in a capable partner who advises you that he can take over the management of the site.

There are probably many online businesses that have stalled and could be expanded with outside help.

And there are an equal number of startup concepts that are actually very straightforward to implement. With patience and good execution, they could be built out profitably.

I believe you should look out for an agency to outsource the critical processes around running the show. The reason being, it would require much more than one partner/individual to actually run the show and trading everything for equity may not be a good idea.

On a separate not, at 366Pi, we, do, manage critical functional processes on behalf of our clients. Do let me know if you find it worthy enough to discuss further.

I presume that is $100k USD per investment, not in total?

Early stage tech investment is a numbers game, in that the rewards are potentially great but the number of failures is high. Startups are defined differently to regular businesses because of their attempt to grow unfeasibly quickly. That's why angel and VC capital is needed, to short-circuit an extended growth time, as a regular business would. Not surprisingly that means risks increase.

With that in mind, understand what sort of investment portfolio you want (and the risk you're willing to assume) will help decide how you invest your money. If you want to advise and even be hands-on, geography will be important.

You could then look at syndicates on Angel List (though these require some consideration as they vary in quality, unless you put your funds in a syndicate of syndicates from a more experienced syndicator), going direct to startups (though this is time and knowledge intensive) or one of the many crowd funding platforms.

There are also people -like myself- who have clubs or networks of high-net worths looking to invest on deals together. Happy to discuss further if you wish.