Apart from the 100MHz processor (slow, but useable) and the slow motherboard clock, your biggest problem is going to be getting any more memory - the mem used on the old Pentiums is hard to get so I doubt if you'll get anything to fit a 486 (they keep changing the "standards" every time they change processor generations!)
As you have useable hard discs and so forth, and you are capable of "refurbishing", how about fitting a new motherboard with a Celeron or a AMD Duron (nice an cheap!) with at least 64k of "modern" memory. You can keep the HDDs and the floppies, but you may need to get a new case (the old PSUs are not compatible with new motherboards - "old" PSUs can be recognised by two connectors to the motherboard with one row of pins, "new" ones have a single connector with two rows of pins) but case/power supplies are very cheap. Also check if you have the right type of slot available for your old video card.
It won't be "blazing fast", but it'll be okay for your needs. Once you have worked out the budget for the upgrade, compare it with the cost of a new system and, if it's a significant saving go for the upgrade (with the new case/power supply, it'll look like a new box too!)
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What goes around . . .
. . often lands better!