This is a partial answer to the OP's question about the suitability of a Turkish eVisa for their intended trip to Turkey by air, then across Turkey by land, then exiting Turkey though a land crossing into another country.
TL;dr
While the OP can apply online for a Turkish eVisa, the OP does not meet all the eVisa requirements and their application for an eVisa will be denied.
Instead, the OP will need a regular visa issued by a Turkish Embassy or Consulate
The Whole Explanation
The Turkish government's opening page for Turkish eVisas (also cited in my comment to the OP's question) notes that Nepali citizens are eligible to apply for Turkish eVisas.
However, more hurdles exist. Continuing the eVisa application process brings up this page, which lists additional specific requirements for issuance and use of the eVisa. The chart below appears on this page, and lists these additional requirements next to a column of check boxes:
The first box requires the applicant to hold a return ticket, a hotel reservation, and at least $50 (currency not further specified) per day for each day of stay in Turkey. Here, however, the OP plans to transit Turkey by bus and leave overland. The OP will not have a return ticket, and therefore does not meet all the requirements listed in the first box. The OP cannot check this box.
The second box requires the applicant to have a Supporting Document, defined as a visa or residence permit from a Schengen country, USA, UK, or Ireland. The OP may be able to meet this requirement, but insufficient information is revealed in the question to know if the OP can meet this second box requirements.
The third box requires the OP to have a passport valid for at least 6 months from the date they intend to enter Turkey. The OP may be able to meet this requirement, but insufficient information is revealed in the question to know if the OP can meet this second box requirements.
The fourth box indicates the applicant will travel to Turkey by air. The OP meets this requirement, and can check the fourth box.
The fifth box requires the OP to affirm they meet every requirement listed at boxes 1 through 4. Because the OP cannot show a return ticket — one of the requirements listed in Box 1 — the OP does not meet every requirement listed on this form, and cannot check the fifth box.
Summary: the OP cannot check the first and fifth boxes, and may be unable to check the second and third boxes. Because the OP cannot check all five boxes, they are not eligible for a Turkish eVisa for this journey.